Foundations Of Modern Analysis - J. Dieudonne.pdf

f(u)

lini f ( x ) r-a.

xfa

and show that it is at most denumerable, using Problem 2(a) of Section 3.9. Consider similarly the set of points a E E such that f ( x ) < p .= q

lim x-a.


x i a

14. C A U C H Y SEQUENCES, COMPLETE SPACES

In a metric space E, a Caucky sequence is an infinite sequence (x,) such that, for any E > 0, there exists an integer no such that the relations p 3 no and q 3 no imply d(x,, xq) < E .

(3 A4.1)

AH^ convergent sequence

For if a

=

lim x,,, for any ,+a3

E

is

a Cauchy sequence.

> 0 there exists no such that

II

3 no implies

d(a, x,,)< 4 2 ; by the triangle inequality, the relations p 3 n o , q 3 no imply d ( xp,xy) < E .

14 CAUCHY SEQUENCES, COMPLETE SPACES 53

(3.14.2)If (x,) is a Cauchy sequence, any cluster value of (x,) is a limit of (x,). Indeed, if b is a cluster value of (x,), given E > 0, there is no such that p 2 no and q > no imply d(x,, xy) < 4 2 ; on the other hand, by (3.13.11) there is a p o 2 no such that d(b, x p o )< 4 2 ; by the triangle inequality, it follows that d(b, x,) < E for any n 2 n o . A metric space E is called complete if any Cauchy sequence in E is convergent (to a point of E, of course).

(3.14.3)The real line R is a complete metric space. Let (x,) be a Cauchy sequence of real numbers. Define the sequence (n,) of integers by induction in the following way: no = 1 and n k + l is the sma~~estinteger > n, such that, f o r p 3 and q 3 n k + l , Ix, - xyl < I / 2 k + Z ; the possibility of the definition follows from the fact that (x,) is a Cauchy sequence. Let I, be the closed interval [x,, - 2 - , , x,, 2 - k ] ; we have I , + ~c r,, for IX,~ - xnL+,l < 2 - k - ' ,. on the other hand, for m 2 t ? k , x, E 1, by definition. Now from axiom (IV) (Section 2.1) it follows that the nested intervals I, have a nonempty intersection; let a be in I, for all k . Then it is clear that la - x,I d 2 - h + ' for all m 3 n k r hence a = lim x,.

+

n+ m

(3.14.4)Ij'a subspace F of a metric space E is complete, F is closed in E. Indeed, any point a E F is the limit of a sequence (x,) of points of F by (3.13.13). The sequence (x,) is a Cauchy sequence by (3.14.1), hence by assumption converges t o a point b in F; but by (3.13.3) b = a, hence a E F ; this shows F = F. Q.E.D.

(3.14.5)In a complete metric space E, any closed subset F is a complete subspace. For a Cauchy sequence (x,) of points of F converges by assumption to a point a E E, and as the x, belong to F, a E F = F by (3.13.7). Theorems (3.14.4) and (3.14.5) immediately enable one to give examples both of complete and of noncomplete spaces, starting from the fact that the real line is complete.

Ill

54

METRIC SPACES

The fundamental importance of complete spaces lies in the fact that to prove a sequence is convergent in such a space, one needs only prove it is a Cauchy sequence (one also says that such a sequence satisfies the Cauchy criterion); the main difference between application of that test and of the definition of a convergent sequence is that in the Cauchy criterion one does not need to know in advance the oalue of the limit. We have already mentioned that on a same set E, two distances d , , d , may be topologically equivalent, but the identity mapping of E, into E, (El, E, being the corresponding metric spaces) may fail to be uniformly continuous. This is the case, for instance, if we take E = R, d,(x, y ) = Jx- pJ, d l ( x , y ) being the distance in the extended real line, restricted to R; E, is then complete and not El since El is not closed in 8. When two distances d,, d, are such that the identity mapping of El into E, is uniformly continuous as well as the inverse mapping, d, and d, are said to be unijornily equbalent. Cauchy sequences are then the same for both distances. For instance, if there exist two real numbers a > 0, fl > 0 such that, for any pair of points x,y in E, adl(x, y ) < d,(x, y ) < fldl(x,y ) , then (1, and d, are uniformly equivalent distances. Let E, E’ be two metric spaces, A a subset of E , f a mapping of A into E’; the oscillatioti of,f in A is by definition the diameter 6 ( j ( A ) )(which may be a).Let a be a cluster point of A ; the oscillation o j f at the point a \\tit11 respect to A is Q ( a ; f ) = infii(,f( V n A)), where V runs over the set of

+

V

neighborhoods of a (or merely a fundamental system of neighborhoods). (3.14.6) Suppose E’ is a complete metric space; in order that

lim f(x) %*a, x E A

exist, a necessary arid su-cient condition is that the oscillation of ,f at the point a, with respect to A, be 0. The condition is necessary by (3.13.2). Suppose conversely that it is satisfied, and let (x,) be a sequence of points of A converging to a ; then it follows from the assumption that the sequence ( f ( x , ) ) is a Cauchy sequence in E‘, for, given any E > 0, there is a neighborhood V of a such that d ‘ ( f ( x ) , f ( y ) )< E for any two points x, y in V n A , and we have x,,E V n A except for a finite number of indices. Hence the sequence ( f ( x , ) ) has a limit a’. Moreover, for any other sequence (y,) of points of A , converging to a, the limits of ( f ( x , ) ) and of (f(y,,)) are the same since d’(f(x,),f(y,)) < E as soon as x, and yn are both in V n A. Hence lim f ( x ) = a‘ from the definition of the limit and from (3.13.14).

x-a, x E A

15

ELEMENTARY EXTENSION THEOREMS 55

PROBLEMS 1. (a) Let E be an rdtvametvic space (Section 3.8, Problem 4). In order that a

sequence (x,) in E be a Cauchy sequence, show that it is necessary and sufficient that lim d ( x , , ~ , , + ~ ) = O . n- m

(b) Let X be an arbitrary set, E the set of all infinite sequences x = (x,) of elements of X. For any two distinct elements x = (xJ, y = (y.) of E, let k ( x , y ) be the smallest integer 12 such that x, # yn ; let d(x, y ) = I/k(x,y ) if x # y , d ( x , x) = 0. Prove that d is an ultrametric distance on E, and that the metric space E defined by d is complete. 2. Let 'p be an increasing real valued function defined in the interval 0 < u < +a,and such that q(0) = 0, q(u)> 0 if u > 0, and y(ir u ) < p(ir) q ( u ) . Let c/(x, y ) be a distance on a set E ; then d,(x, y ) = q(d(x, y ) ) is another distance on E. (a) Show that if q is continuous at the point [I = 0, the distances d a n d d, are uniformly equivalent. Conversely, if, for the distanced, there is a point xo E E which is not isolated in E (3.10.10),and if ciand d, are topologically equivalent, then q is continuous at the point I I = 0. (b) Prove that the functions

+

I!'

(0 < r

< l),

log(1

+

I/),

u/(I

+

+

ti),

inf(1, u )

satisfy the preceding conditions. Using the last two, it is thus seen that for any distance on E, there is a uniformly equivalent distance which is borinded. 3. On the real line, let d(x, y ) = Ix - y / be the usual distance, d'(x, y ) = / x 3 - y31 ; show that these two distances are topologically equivalent and that the Cauchy sequences are the same for both, but that they are not uniformly equivalent. 4. Let E be a complete metric space, d the distance on E, A the intersection of a sequence (U,) of open subsets of E ; let F, = E - U,, and for every pair of points x , y of A, write

d h , Y ) =L(x, y)l(l + f ; d x , Y ) ) , and d'(x, Y ) = d(x, Y )

+ C d d x , y)/2". Show that on m

Il=O

the subspace A of E, d' is a distance which is topologically equivalent to d , and that for the distance d',A is a complete metric space. (Note that a Cauchy sequence for d' is also a Cauchy sequence for d, but that its limit in E may not belong to any of the F, .) Apply to the subspace I of R consisting of all irrational numbers.

15. EL EM E N T A R Y E X T E N S I 0 N TH E OREMS

(3.15.1) Lei f and g be two continuous mappings of a metric space E into a metric space E'. The set A of the points x E E such that f ( x ) = g(x) is closed iti E. It is equivalent to prove the set E - A open. Let a E E - A, then f ( a ) # g(a); let d'( f (a), g(a)) = a > 0. By continuity o f f , g at a and from (3.6.3) it follows that there is a neighborhood V of a in E such that for

56

Ill METRIC SPACES

x E V, d’( f (a), f ( x ) ) < 4 2 and d’(g(a),g(x)) < a/2. Then for x E V, f ( x ) # g(x), otherwise we would have d‘(f(a),g(a)) < c( by the triangle inequality. (“ Principle of extension of identities ”) Let f, g be two contin(3 .I5.2) uous mappings of a metric space E into a metric space E’; if f ( x ) = g ( x ) for all points x of a dense subset A in E, then f = g.

For the set of points x where f ( x ) = g(x) is closed by (3.15.1) and contains A. (3.15.3) Let f, g be two continuous mappings of a metric space E into the The set P of the points x E E such that f ( x ) < g ( x ) is extended real line closed in E.

w.

We prove again E - P is open. Supposef(a) > g(a), and let p E R be such that f ( a ) > p > g(a) (cf. (2.2.16) and the definition of R in Section 3.3). The inverse image V b y j o f the open interval ]p, + 001 is a neighborhood of a by (3.11 .I); so is the inverse image W by g of the open interval [ - 00, p[. Hence V n W is a neighborhood of a by (3.6.3), and for x E V n W, f(x) > p > g(x). Q.E.D. (3.15.4) (“ Principle of extension of inequalities”) L e t f , g be two continuous mappings of a metric space E into the extended real line R; i f f ( x ) < g(x) for all points x of a dense subset A of E, then f ( x ) < g(x)for all x E E.

The proof follows from (3.1 5.3) as (3.1 5.2) from (3.1 5.1). (3.15.5) Let A be a dense subset of a metric space E, and f a mapping of A into a metric space E‘. In order that there exist a continuous mapping f of E into E’, coinciding with f in A, a necessary and suficient condition is that, for any x E E, the limit lim f ( y ) exist in E’; the continuous mapping f is Y+X,Y EA then unique.

As any x E E belongs to A, we must have f ( x ) = lim f ( y ) by (3.13.5), Y-X,YEA

hencef(x) = lirn f ( y ) ; this shows the necessity of the condition and the Y+X, Y E A

fact that if the continuous mapping J exists, it is unique (this follows also

16 COMPACT SPACES

57

from (3.1 5.2)). Conversely, suppose the condition satisfied, and let us prove that the mappingfdefined byf(x) = lim f ( y ) is a solution of the extension Y-x, Y FA

problem. First of all, if x E A, the existence of the limit implies by definition f(x) = f(x), hence f extends f, and it remains to see that f is continuous. Let x E E, V' a neighborhood off(x) in E'; there is a closed ball B' of center f (x) contained in V'. By assumption, there is an open neighborhood V of x in E such thatf(V n A) c B' (by (3.13.1)). For any y E V,f(y) is the limit of f a t the pointy with respect to A, hence also with respect to V n A, by (3.13.5); hence, it follows from (3.13.7) that f(y) E f(V n A), and therefore f ( y ) E B' since B' is closed. Q.E.D.

(3.15.6) Let A be a dense subset of a metric space E, and f a uniformly continuous mapping of A into a complete metric space E'. Then there exists a continuous mapping f of E into E' coinciding with f i n A; moreover, f is uniformly continuous.

To prove the existence off, it follows from (3.15.5) and (3.14.6) that we have to show the oscillation of f at any point x E E, with respect to A, is 0. Now, for any E > 0, there is 6 > 0 such that d(y, z) < 6 implies d'(f(y), f(z)) < 4 3 (y, z in A). Hence, the diameter of f ( A n B(x; 612)) is at most ~ / 3 which , proves our assertion. Consider now any two points s, t in E such that d(s, t ) c 612. There is a Y E A such that d(s, y) < 614 and d ' ( f ( s ) ,f(y)) c ~ / 3 and , a z E A such that d(t, z) < 614 and d'Cf(t),f(z)) < 4 3 . From the triangle inequality it follows that d(y, z) < 6, and as y , z are in A, d'(f (y),f (z)) < 4 3 ; hence, by the triangle inequality, d'(f(s),3(t))< E ; this proves that f is uniformly continuous.

PROBLEM

Let n + r. be a bijection of N onto the set A of all rational numbers x such that 0 < x < 1 (2.2.15). We define a function in E = [0, I ] by puttingf(x) = 1/2", the infinite sum being ,"<%

extended only to those n such that r,, < x. Show that the restriction o f f t o the set B of all irrational numbers x E [0, I ] is continuous, but cannot be extended to a function continuous in E.

16. COMPACT SPACES

A metric space E is called compact if it satisfies the following condition (" Borel-Lebesgue axiom ") : for every coiiering ( UA)AEof E by open sets

58

111 METRIC SPACES

(“ open covering”) there exists a j n i t e subfamilj) (UA)iEI I (H c L and finite) n.hich is a covering of E. A metric space E is called precompact if it satisfies the following condition:for any E > 0, there is aJinite covering of E by sets of diameter < E . This

is immediately equivalent to the following property: .for afiy E > 0, there is a finite subset F of E such that d(x, F) < cfor every x E E. In the theory of metric spaces, these notions are a substitute for the notion of “finiteness” in pure set theory; they express that the metric space is, so t o speak, “ approximately finite.” Note that, from the definition, it follows that compactness is a topological notion, but precompactness is not (see remark after (3.17.6)).

(3.16.1) For a metric space E, the following three conditioiis are equivalent: (a) E is compact; (b) any infinite sequence in E has at least a cluster value; (c) E is precompact and coniplete.

(a) * (b): Let (x,) be an infinite sequence in the compact space E, and let F, be the closure of the set {x,, x , + ~ ., . . , x,,+,,, . . .}. We prove there is a point belonging to all F,. Otherwise, the open sets U, = E - F, would form a covering of E, hence there would exist a finite number of them, UnI,. . . , U,,, forming a covering of E; this would mean that F,, n F,, n ... n F,, = 0; but this is absurd, since if n is greater than max(n,, . . . , nk), F, (which is not empty by definition) is contained in all the F,,,(1 < i < k). Hence the intersection

m

n= 1

F, contains at least a point a. By (3.13.11) and the definition of a

cluster point, a is a cluster value of (x,,). (b) * (c): First any Cauchy sequence in E has a cluster value, hence is convergent by (3.14.2), and therefore E is complete. Suppose E were not precompact, i.e. there exists a number c( > 0 such that E has no finite covering by balls of radius a. Then we can define by induction a sequence (x,) in the following way: x1 is an arbitrary point of E; supposing that d(x,, x,) 2 ct for i Zj,1 < i < n - I , I < ,j < n - 1, the union of the balls of center x i (1 < i < n - I) and radius ct is not the whole space, hence there is x, such that d ( x i ,x,) 2 c( for i < 17. The sequence (x,) cannot have a cluster value, for if a were such a value, there would be a subsequence (x,J converging t o a, hence we would have d(a, x n k )< 4 2 for k > ko , and therefore d(x,, , x,J < ct for h 3 k, , k 3 k , , 12 # k , contrary t o the definition of (x,,). (c) * (a): Suppose we have an open covering (U,JiELof E such that no finite subfamily is a covering of E. We define by induction a sequence (B,) of

16 COMPACT SPACES 59

balls in the following way: from the assumption it follows that the diameter of E is finite, and by multiplying the distance on E by a constant, we may assume that 6(E) < 1/2,hence E is a ball B, of radius 1. Suppose the B, have been defined for 0 < k < n - 1, and that for these values of k , B, has a radius equal to 1/2,,and there is no finite subfamily of (U,),eL which is a covering of B, . Then we consider a finite covering (vk),,,,, of E by balls of radius 1/2”; among the balls vk which have a nonempty intersection with B,,-,, there is one at least B, for which no finite subfamily of (U,) is a covering; otherwise, as these V , form a covering of B,-,, there would be a finite subfamily of (U,) which would be a covering of B,-l ; the induction can thus proceed indefinitely. Let x, be the center of B,; as B,-l and B, have a common point, the triangle inequality shows that

x,) Hence, if n

< 1 p - 1 + 1/2” < 1/2,-2

< p < q, we have

This proves that (x,,) is a Cauchy sequence in E, hence converges to a point a. Let 1, be an index such that a E Un0;there is an a > 0 such that B ( a ; a ) c U,, . From the definition of a, it follows there exists an integer n such that d(a, x,) < 4 2 , and l/2”< 42. The triangle inequality then shows that B, c B ( a ; a ) c Ulo. But this is a contradiction since no finite subfamily of (U,) is supposed to be a covering of B,, .

(3.16.2) Any precompact metric space is separable. If E is precompact, for any n there is, by definition, a finite subset A, of E such that for every x E E, d(x, A,,) < l/n. Let A = A,; A is at most denumerable, and for each x E E, d(x, A) d(x, A) = 0, E =A.

< d(x, A,)

u n

< l/n for any n, hence

(3.16.3) Let E be a metric space. A n y two of the following properties imply the third: (a) E is compact. (b) E is discrete (more precisely, homeomorphic to a discrete space). (c) E isfinite.

60

Ill METRIC SPACES

(a) and (b) imply (c), for each one-point set {x} is open, hence the family of sets {x} is an open covering of E, and a finite subfamily can only be a covering of E if E is finite. On the other hand, (c) implies both (a) and (b), for each one point set being closed, every subset of E is closed as finite union of closed sets, hence every subset of E is open, and therefore E is homeomorphic to a discrete space. Finally, as there is only a finite number of open sets, E is compact.

(3.16.4) In a compact metric space E, any infinite sequence (x,) which has only one cluster value a converges to a. Suppose a is not the limit of (x,); then there would exist a number a > 0 such that there would be an infinite subsequence (x,,) of (x,) whose points belong to E - B(a; a). By assumption, this subsequence has a cluster value b , and as E - B(a; a ) is closed, b belongs to E - B(a; a ) by (3.13.7). The sequence (x,) would thus have two distinct cluster values, contrary to assumption.

(3.16.5) Any continuous mapping f of a compact metric space E into a metric space E‘ is uniforndy continuous. Suppose the contrary; there would then be a number a > 0 and two sequences (x,) and (y,) of points of E such that d(x,, y,) < I/n and d’(f(x,),f ( y , ) ) 3 a. We can find a subsequence (x,) converging to a point a, and as d(x,,, y,,) < l/nk,it follows from the triangle inequality that the sequence (ynL)also converges to a. But f is continuous at the point a, hence there is a 6 > 0 such that d ’ ( f ( a ) , f ( x ) ) < a / 2 for d(a, x) < 6. Take k such that d(a, x,) < 6, d(a, y n k )< 6; then d’(f(x,,),f(y,,)) < a contrary to the definition of the sequences (x,) and (y,).

(3.16.6) Let E be a compact metric space, (U,),ELan open covering of E. There exists a number a > 0 such that any open ball of radius a is contained in at least one of the U, (“ Lebesgue’s property ”). For every x E E, there exists an open ball B(x; r,) contained in one of the sets U,. As the balls B(x; r,/2) form an open covering of E, there exist a finite number of points x i E E such that the balls B(x,; r J 2 ) form a covering of E. If a > 0 is the smallest of the numbers rxi/2,it satisfies the required property: indeed, every x E E belongs to a ball B(x,; rJ2) for some i, hence B(x; a) is contained in B(x,; rXi)since a < r x i / 2 ;but by construction B(x,; r X i ) is contained in some U,.

17 COMPACT SETS 61 PROBLEMS 1. Give an example of a precompact space in which the result of (3.16.6) fails to be true. 2. For a metric space E, show that the following properties are equivalent: (a) E is compact; (b) every denumerable open covering of E contains a finite subcovering; (c) every decreasing sequence (F,) of nonempty closed sets of E has a nonempty intersection; (d) for any infinite open covering ( U A ) A eof, ~E, there is a subset H c L, distinct from L and such that (UA),." is still a covering of E; (e) every poititwisefinire open covering (U,) of E (i.e. such that for any point x E E, x E UA only for a finite subset of indices) contains a finite subcovering; (f) every infinite subspace of E which is discrete is not closed. (Using (3.16.1), show that (f) implies (a), and that (d) and (e) imply (f).) 3. Let E be a metric space, d the distance on E, %(E) c $(E) the set of all closed nonempty subsets of E. We may suppose that the distance on E is bounded (Section 3.14, Problem 2). For any two elements A, B of S(E), let p(A, B) = sup d(x, B), h(A, B) = X€A sup(p(A, B), p(B, A)). (a) Show that, on g(E), h is a distance (the "Hausdorff distance"). (b) Show that for any four elements A, B, C, D of g(E), one has

h(A u B, C u D) < max(h(A, C), h(B, D)). (c) Show that if E is complete, 8(E) is complete. (Let (X.) be a Cauchy sequence in s(E); for each n, let Y , be the closure of the union of the sets X,,, such that p 2 0; consider the intersection of the decreasing sequence (Y.) in E.) (d) Show that if E is precompact, %(E) is precompact (use the problem in Section 1.1). Therefore, if E is compact, 8(E) is compact. 4. Let E be a compact metric space. For every E > 0, let N,(E) be the smallest integer n such that there exists a covering of E by n sets of diameter < 2.5; let M,(E) be the largest integer m such that there exists a finite sequence of m points of E for which the distance of any two (distinct) of these points is > 8 . The number H,(E) = log N,(E) is called the &-entropy of E, the number C,.(E) = log MJE) the &-capacity of E. (a) Show that M,,(E) < N,(E) < M,(E), hence C,,:(E) < H,(E) < C,(E). (b) Show that the functions N,(E) and MJE) of E , defined for 8 > 0, are decreasing and continuous on the right (to prove the continuity of N,(E) on the right, use contradiction, and apply problem 3(d)). (c) If A and B are closed nonempty subsets of E, show that NdA

LJ

M A LJ

+N O ) , B) < HAA) + H,(B), B) < N A N

M,(A

LJ

B) < MAA)

CAA LJ B) < CAA)

+ MAB)

+ CdB).

(d) If E is a closed interval of R of length I , show that NJE) = M2,(E) = 1/28 if / / 2 is~ an integer, and N,(E) = M,,(E) = [//2.5] 1 (where [ I ] is the largest integer < t for t > 0) if / / 2 ~is not an integer.

+

17. COMPACT SETS

A compact (resp. precompact) set in a metric space E is a subset A such that the subspace A of E be compact (resp. precompact).

62

111 METRIC SPACES

(3 .I 7.1)

Any precompact set is bounded.

This follows from the fact that a finite union of bounded sets is bounded (3.4.4). The converse of (3.17.1) does not hold in general, for any distance is equivalent to a bounded distance (Section 3.14, Problem 2) (but see (3.17.6)).

(3.17.2)

Any compact set in a metric space is closed.

Indeed, such a subspace is complete by (3.16.1), and we need only apply (3.14.4).

(3 .I 7.3) In a compact space E, every closed subset is compact.

For such a set is obviously precompact, and it is a complete subspace by (3.14.5).

A relatively compact set in a metric space E is a subset A such that the closure A is compact. (3.17.4) Any subset of a relatively compact (resp. precompact) set is relatioely compact (resp. precompact).

This follows at once from the definitions and (3.17.3).

(3.17.5) A relatively compact set is precompact. In a conlplete space, a precompact set is relatively compact.

The first assertion is immediate by (3.17.4). Suppose next E is complete and A c E precompact. For any E > 0, there is a covering of A by a finite number of sets c, of A having a diameter < ~ / 2 each ; ck is contained in a closed ball D, (in E) of radius 4 2 . We have therefore A c D,, the set

u D, k

u

being closed, and each D, has a diameter

k

< E . On the other hand, A is a

complete subspace by (3.14.5), whence the result. A precompact space E which is not complete gives an example of a precompact set which is not relatively compact in E.

17 COMPACT

SETS 63

(3.17.6) (Borel-Lebesgue theorem). In order that a subset of the real line be relatively compact, a necessary and suficient condition is that it be bounded. In view of (3.17.1), (3.17.4), and (3.17.5), all we have to do is to prove any closed interval [a, b] is precompact. For each integer n, let xk = a k(b - a)/n (0 < k d I ? ) ; then the open intervals of center xk and length 2/n form a covering of [a, b]. Q.E.D.

+

Remark. If, on the real line, we consider the two distances d,, d, defined in Section 3.14, it follows from (3.17.1) that E, is not precompact, whereas El is precompact, since the extended real line R, being homeomorphic to the closed interval [- 1, + I ] of R (3.12), is compact by (3.17.6). (3.17.7) A necessary and sufJjcient condition that a subset A of a metric space E be relatively compact is that every sequence of points of A have a cluster value in E. The condition is obviously necessary, by (3.16.1). Conversely, let us suppose it is satisfied, and let us prove that every sequence (x,) of points of A has a cluster value in E (which will therefore be in A by (3.13.7)), and hence that A is compact by (3.16.1). For each n, it follows from the definition of closure that there exists y , E A such that d(x,, y,) d Ijn. By assumption, there is a subsequence (y,J which converges to a point a ; from the triangle inequality it follows that (x,,) converges also to a. Q.E.D. (3.17.8) The union of two relatiuely compact sets is relatively compact. From (3.8.8) it follows that we need only prove that the union of two compact sets A, B is compact. Let (UJdELbe an open covering of the subspace A u B ; each U, can be written (A u B) n V,, where V, is open in E, by (3.10.1). By assumption, there is a finite subset H (resp. K) of L such that the subfamily(A n V,)neb,(resp. (6 n VA)AeK)is a covering of A (resp. B). It is then clear that the family ((A n B) n V,), E H is a covering of A u B. (3.17.9) Let f be a continuous mapping of a metric space E into a metric space E‘. For every compact (resp. relatiuely compact) subset A of E, ,f(A) is compact, hence closed in E’ (resp. relatively cornpact in El).

64

Ill

METRIC SPACES

It is enough t o prove that f ( A ) is compact when A is compact. Let ( U l ) l E Lbe an open covering of the subspace ,f(A) of E'; then the sets A nf -'(UA) form an open covering of the subspace A by (3.11.4); by assumption, there is a finite subset H of L such that the sets A n , f - ' ( U L ) for I E: H still form a covering of A ; then the sets U, =f(A n f - ' ( U L ) ) for ;IE H will form a covering of,f(A). Q.E.D.

(3.17.10) Let E be a nonetiipty compact metric space, f a continuous mapping of E into R; tliett,f(E) is bounded, and there exist tlt'o points a , h in E such that . f @ ) = inf,fW,f(b) = sup.f(.4. xeE

XE

E

The first assertion follows from (3.17.9) and (3.17.1). On the other hand, f(E) is closed in R by (3.17.2), hence supf(E) and inff(E), which are cluster points off(E), belong to,f(E).

(3.17.1 1) Let A be a conipact subset in a metric space E. Then the sets V,(A) (see Section 3.6) f o r m a fundatiiental system of neigliborhoods of A. Let U be a neighborhood of A ; the real function x + d(x,E - U) is > 0 and continuous in A by (3.11.8), hence there is a point xo E A such that d(xo,E - U) = inf d(x, E - U) by (3.17.10). But d(xo,E - U) = r > 0; X E A

hence V,(A) c U.

(3.17.12) I f E is a compact nietric space, f a continuous injectii'e tilapping of E into a metric space E', then f is a lrotiieotiiorphism of' E onto f (E). All we need to prove is that for every closed set A c E, ,f(A) is closed inf(E) (by (3-11.4)); but this follows from (3.17.3) and (3.17.9).

PROBLEMS

1. Let f be a uniformly continuous mapping of a metric space E into a metric space E'. Show that for any precoinpact subset A of E,f(A) is precompact. 3. In a metric space E, let A be a compact subset, B a closed subset such that A n B = 0. Show that d(A, B) > 0.

18 LOCALLY COMPACT SPACES 65 3. Let E be a compact ultrametric space (Section 3.8, Problem 4), d the distance on E. Show that for every xo E E, the image of E by the mapping .Y -+cf(xo, x ) is an at most denumerable subset of the interval [0, +a[in which every point (with the possible exception of 0) is isolated (3.10.10).(For any I’ = d(xo,x ) > 0, consider the 1.u.b. of cl(xo, y ) on the set of points y such that d(x,,, y ) < 1’, and the g.1.b. of d(xo,z) on the set of points z such that c/(x,, z) > r; use Section 3.8,Problem 4). 4. Let E be a compact metric space, d the distance on E,fa mapping of E into E such that, for any pair (x, y ) of points of E, d(f(x),f(y)) 2 d(x, y). Show thatfis an isometry ofE onto E. (Let a, b be any two points of E ; put fn =fn-, o f , a, =fn(a), b, =f,(b); show that for any E > 0 there exists an index k such that d(a, ak) C E and d(b, bk) < E (consider a cluster value of the sequence (a“)), and conclude that f(E) is dense in E and that 4 f ( a ) , f ( b ) )= 4 0 , b).) 5. Let E, E’ be two metric spaces,fa mapping of E into E’. Show that if the restriction off to any compact subspace of E is continuous, thenfis continuous in E (use (3.13.14)). 6 . Let E,E’ be two metric spaces, f a continuous mapping of E into E , K a compact subset of E. Suppose the restriction f l K off is injective and that for every x E K, there is a neighborhood V, of x in E such that the restriction f 1 V, o f f is injective. Show that there exists a neighborhood U of K in E such that the restriction f 1 U is injective (use contradiction and (3-17.11)).

18. LOCALLY COMPACT SPACES

A metric space E is said to be locally conipact if for every point x E E, there exists a compact neighborhood of x in E. Any discrete space is locally compact, but not compact unless it is finite (3.16.3).

(3.18.1) The real line R is locally conipact hut not compact. This follows immediately from the Borel-Lebesgue theorem (3.17.6).

(3.18.2) Let A be a conipact set in a locally compact metric space E. Then there exists an r > 0 such that V, (A) (see Section 3.6) is relatitlely conzpact in E. For cach x E A, there is a compact neighborhood V, of x ; the 9, form an open covering of A, hence there is a finite subset {xl, . . . , x,} in A such that the

TX,(1 < i 6 n) form an open covering of A. The set U =

u n

i= 1

V,: is com-

pact by (3.17.8) and is a neighborhood of A; hence the result, by applying (3.17.11).

66

Ill METRIC SPACES

(3.18.3) Let E be a locally compact metric space. The following properties are equivalent: (a) there exists an increasing sequence (U,) of open relatively conipact c Un+l for every 11, and E = U,; sets in E such that

u

n,

n

(b) E is a denurnerable uriion of compact subsets; (c) E is separable.

u,

It is clear that (a) implies (b), since is compact. If E is the union of a sequence (K,) of compact sets, each subspace K, is separable (by (3.16.2)); if D, is an at most denumerable set in K,, dense with respect to K,,, then D = D, is at most denumerable, and dense in E, since E =

u n

K, c

u u 15, n

c

15; hence (b) implies (c). Let us suppose finally that E

fl

is separable, and let (T,) be an at most denumerable basis for the open sets of E (see (3.9.4)). For every x E E, there is a compact neighborhood W, of x, hence, by (3.9.3), an index M ( X ) such that x E Tnc,) c W,. It follows that those of the T, which are relatively compact already constitute a basis for the open sets of E. We can therefore suppose that all the T, are relatively compact. We then define U, by induction in the following way: U1 = TI, U n + , is the union of Tntl and of V,(U,), where r > 0 has been taken such that V,(u,) is relatively compact (which is possible by (3.18.2)); it is then clear that the sequence (U,) verifies property (a). (3.18.4) In a locally compact metric space E, eriery opeii subspace atid every closed subspace is locally compact. Suppose A is open in E; for every a E E, there is a closed ball B’(a; r ) which is compact (from the definition of a locally compact space and (3.17.3)). On the other hand, there is r‘ Q r such that the ball B’(a; r’) is contained in A ; as it is compact by (3.17.3), A is locally compact. Suppose A is closed in E, and let a E A ; then, if V is a compact neighborhood of a in E, V n A is a neighborhood of a in A by (3.10.4), and is compact by (3.17.3); this proves A is locally compact.

PROBLEMS 1. If A is a locally compact subspace of a metric space E, show that A is locally closed (Section 3.10, Problem 3) in E. The converse is true if E is locally compact (use (3.18.4)). 2. (a) Show that in a locally compact metric space, the intersection of two locally com-

pact subspaces is locally compact (cf. Problem I).

19 CONNECTED SPACES AND CONNECTED SETS 67

(b) In the real line, give an example of two locally compact subspaces whose union is not locally compact, and an example of a locally compact subspace whose complement is not locally compact. 3. (a) Give an example of a locally compact metric space which is not complete. (b) Let E be a metric space such that there exists a number r > 0 having the property that each closed ball B’(x; r ) ( x E E) is compact. Show that E is complete and that for any relatively compact subset A of E, the set V:,,(A) of the points x E E such that d(x, A) < rj2 is compact.

19. CONNECTED SPACES A N D CONNECTED SETS

A metric space E is said to be connected if the only subsets of E which are both open and closed are the empty set /zr and the set E itself. An equivalent formulation is that there does not exist a pair of open nonempty subsets A, B of E such that A u B = E, A n B = /zr. A space reduced to a single point is connected. A subset F of a metric space E is connected if the subspace F of E is connected. A metric space E is said to be locally connected if, for every x E E, there is a fundamental system of connected neighborhoods of+.

(3.19.1) In order that a subset A of the real line R be connected, a necessary and suficient condition is that A be an interval (bounded or not). The real line is a connected and locally connected space. The second assertion obviously follows from the first. Suppose A is connected; if A is reduced to a single point, A is an interval. Suppose A contains two distinct points a < 6. We prove every x such that a < x < b belongs to A. Otherwise, A would be the union of the nonempty sets B = A n ] - c o , x [ and C = A n ] x , +a[, both of which are open in A and such that B n C = From this property, we deduce that A is necessarily an interval. Indeed, let c E A, and let p , q be the g.1.b. and 1.u.b. of A in R ; if p = -a,then for every x < c, there is y < x belonging to A ; hence x E A, so ] - co, c] is contained in A; if p is finite and p < c, for every x such that p x < c there is y E A such that p < y < x, hence again x E A, so that A contains the interval ] p , c]. Similarly, one shows that A contains [c,q[ if q > c ; it follows that in any case A contains the interval ]p,q [ , and therefore must be one of the four intervals in R of extremities p , q (of course, if p = - 00 (resp. q = 00) p (resp. q ) does not belong to A). Conversely, suppose A is a nonempty interval of origin a and extremity b in R (the possibilities a = - co, n 4 A, b = + co, b 4 A being included).

a.

-=

+

68

I l l METRIC SPACES

Suppose A = B LJ C, with B, C nonempty open sets in A and B n C = 0; suppose for instance x E B, y E C, and x < y . Let z be the 1.u.b. of the bounded set B n [x, y ] ; if z E B, then z < y and there is by assumption an interval [ z , z + h [ contained in [x,y ] and in B, which contradicts the definition of z ; if on the other hand z E C, then x < z , and there is similarly an interval ] z - h, z ] c C n [x,y ] , which again contradicts the definition of z (see (2.3.4));hence z cannot belong to B nor to C, which is absurd since the closed set [x, y ] is contained in A. Hence A is connected.

(3.19.2) rf' A is a connected set in a nietvic space E, then any set B such that A c B c A is connected.

For suppose X, Y are two nonempty open sets in B such that X u Y = B, X n Y = @; as A is dense in B, X n A and Y n A are not empty, open in A, and wewouldhave(X n A) LJ (Y n A) = A , ( X n A) n (Y n A) = 0, a contradiction.

(3.19.3) 111 a metric space E, let (A,), be a family of connected sets haoing a nonempty igtersection; theti A = A, is connected.

u

,EL

Let a be a point of

n A,, and suppose A

=B

v C, where B, C are non-

LEL

empty open sets in A such that B n C = @. Suppose for instance a E 0 ; by assumption there is at least one A such that C n A, # 0; then B n A, and C n A, are open in A, and such that (B n A,) u (C n A,) = A,, (B n A,) n (C n A,) = 0, a contradiction since B n A, # @.

(3.19.4) Let ( A i ) l f i s nbeasequence of coniiected sets such that A i n

0for 1 < i d n - 1;

then

u Ai n

#

is connected.

i= 1

This follows at once from (3.19.3)by induction on n. From (3.19.3)it follows that the union C ( x )of allconnected subsets of E containing a point x E E is connected, hence the largest connected set containing x; it is called the connected component qf x in E. It is clear that for any y E C(x), we have C ( y ) = C(x), and if y $ C(x), then C(x) n C ( y ) = 0; moreover, it follows from (3.19.2)that C(x) is closed in E. For any subset A of E, the connected components (in the subspace A) of the points of the subspace A are called the connected coniponents of A ; if every connected component of A is reduced to a single point, A is said to be totally disconnected.

19 CONNECTED SPACES AND CONNECTED SETS 69

A discrete space is totally disconnected; the set of rational numbers and the set of irrational numbers are totally disconnected, by (2.2.16) and (3.19.1). (3.19.5) In order that a nietric space E be locally connected, a necessary and suflcient condition is that the connected coniponents of the open sets in E be open. The condition is sufficient, for if V is any open neighborhood of a point E, the connected component of x in the subspace V is a connected neighborhood of x contained in V, hence E is locally connected. Thecondition is necessary, for if E is locally connected and A is an open set in E, B a connected component of A, then for any x E B, there is by assumption a connected neighborhood V of x contained in A, hence V c B by definition of B, and therefore B is a neighborhood of every one of its points, hence an open set. x

E

(3.19.6) Any nonernpty opeti set A in the real line R is the union of an at most denumerable .family of' open interoals, ti0 two of which have common points. From (3.19.1) and (3.19.5) it follows that the connected components of A are intervals and open sets, hence open intervals. The intersection A n Q of A with the set Q of rational numbers is denumerable, and each component of A contains points of A n Q by (2.2.16); the mapping Y -+ C(r) of A n Q into the set 0.of the connected components of A is thus surjective, and therefore, by (1.9.2), 0.is at most denumerable. (3.19.7) Let f be a continuous niapping of E into E'; for any connected E, f(A) is connected.

subset A of

Suppose f ( A ) = M u N, where M and N are nonempty subsets of f(A), then, by (3.11.4), A n f -'(M) open in f(A) and such that M n N = 0; and A n f -'(N) would be nonempty sets, open in A and such that A = (A nf -'(M))u(A n f-'(N))and(A n f - ' ( M ) ) n ( A n f-'(N)) = contrary to assumption.

a,

(3.19.8) (Bolzano's theorem) Let E be a connected space, f a continuous niapping of E into the real line R. Suppose a, b are two points of f(E) such

70

Ill

METRIC SPACES

tliat a < h. Theti, f o r

anj’

c such tliat a < c < h tkere exists x

E

E such that

f ( x ) = c.

Forf(E) is connected in R by (3.19.7), hence an interval by (3.19.1). (3.19.9) Let A be a subset of a metric space E. If‘ B is a coiinecred siibset of E such that both A n B arid (E - A) n B are not empty, theii (fr(A)) n B is not empty. In particular, if E is connected, any subset A o j E u‘istiiict f i w r r E atid 0has at least oiie froiitier point. Suppose (fr(A)) n B = 0; let A‘ = E - A ; as E is the union of A, and fr(A), B would be the union of U = A n B and V = A’ n B, both of which are open in B and not empty by assumption (for a point of A n B must belong to A n B since fr(A) n B = 0, and similarly for A‘ n B); as U n V 0, =this would be contrary t o the assumption that B is connected.

A’

Remark. If we agree to call “curve” the image of an interval of R by a continuous mapping (see Section 4.2, Problem 5), (3.19.7) shows that a “curve” is connected, and (3.19.9) that a “curve” linking a point of A and a point of E - A meets fr(A), which corresponds to the intuitive idea of connectedness” (see Problem 3 and Section 5.1, Problem 4). “

PROBLEMS

1. Let E be a connected metric space, in which the distance is not bounded. Show that in

E every sphere is nonenipty. (a) Let E be a compact metric space such that in E, the closure of any open ball B(u: r ) is t h e closed ball B’(a; r ) . Show that in E any open ball B(a; 1.) is connected. (Suppose B(a; r ) is the union C D of two noncmpty sets which are open in B(u: r ) and if ci E C , consider a point x F D such that the distance
--

:

20

4. 5.

6.

(b) Let t + ( f ( t ) g, ( t ) ) be a continuous mapping of the interval [O, 11 into E (fand g being continuous). Show that f i s constant. (If there exist points to E [O, I ] such that g ( t d < 0, consider the open subset U c [0, I ] consisting of all t such that g ( t ) < 0, and use (3.19.6).) I n other words, there are pairs of distinct points in E which “cannot be joined by a curve in E.” In a metric space E, let A and B be two connected sets such that A n B # @; show that A u B is connected. Let A and B be two nonempty subsets of a metric space E. Show that if A and B are closed, A U B and A n B connected, then A and B are connected. Show by an example in the real line that the assumption that both A and B are closed cannot be deleted. Let E be a connected metric space having at least two points. (a) Let A be a connected subset of E, B a subset of A, which is open and closed with respect to A ; show that A U B is connected (apply Problem I of Section 3.10 to the two sets A u B and A). (b) Let A be a connected subset of E, B a connected component of [ A ; show that B is connected (apply (a), using an indirect proof). (c) Show that there are in E two nonempty connected subsets M, N such that M u N = E, M n N = @ (use (b)). In a denumerable metric space E, show that each point has a fundamental system of neighborhoods which are both open and closed. (a) In a metric space E, the connected Component of a point x is contained in every open and closed set containing x. (b) In the plane RZ,let A, he the set of pairs (I//?, y ) such that - 1 < y < 1, B the set of pairs (0, y ) such that 0 -1y < I , C the set of pairs (0, y ) such that - I < y < 0; let E be the subspace of RZ,union of B, C and t h e A, for n > I . Show that E is a locally compact subspace of E, which is not locally connected; the connected components of E are B, C and the A,, 0 7 3 I ) , but the intersection of all open and closed sets containing a point of B is B u C. Let E be a locally compact metric space. (a) Let C be a connected component of E which is compact. Show that C is the intersection of all open and closed neighborhoods of C. (Reduce the problem to the case in which E is compact, using (3.18.2). Suppose the intersection B of all open and closed neighborhoods of C is different from C; B is the union of two closed sets M 3 C and N without common points. Consider in E two open sets U 3 M and V 3 N without common points (Section 3.11, Problem 3). and take the intersections of E - (M u N) with the complements of the open and closed neighborhoods of C.) (b) Suppose E is connected, and let A be a relatively compact open subset of E. Show that every connected component of A has at least a cluster point in A (if not, apply (a) to such a component, and get a contradiction). (c) Deduce from (b) that for every compact subset K of E, the intersection of a connected component of K with E - K is not empty.

c

7. 8.

9.

PRODUCT OF TWO METRIC SPACES 71

c

c

c

c

-_.

20. P R O D U C T O F TWO M E T R I C SPACES

Let El, E, be two metric spaces, d,, dz the distances on El and E, . For any pair of points x = (xl, x,), y = (yl, yz) in E = El x E, , let

4 x ,v>= max(d,(x,, A),4(xZ, yZ)>.

72

111

METRIC SPACES

It is immediately verified that this function satisfies the axioms (I) to (1V) in Section 3.1, in other words, it is a distance on E ; the metric space obtained by taking d as a distance on E is called the product of the two metric spaces El, E, . The mapping (x,,x,) 4 (x, , sl) of El x E, onto E, x El is an isometry. We observe that the two functions d’, d” defined by

d‘(x,Y>= 4(.%Y l )

+ dz(x2

d ” k v) = ((dl(X1, Y l V

9

Yz)

+ (CJ2(x,

9

Y2)>2>”2

are also distances on E, as is easily verified, and are unifornily equioalerit to d (see Section 3.14),for we have

d(x,I’) d d”(x, y ) < d’(x, 4’) < 2 4 % Y ) . For all questions dealing with topological properties (or Cauchy sequences and uniformly continuous functions) it is therefore equivalent to take on E any one of the distances d, d’, d”. When nothing is said to the contrary, we will consider on E the distance d. Open (resp. closed) balls for the distances d, d,, d, will be respectively written B, B,, B, (resp. B’, B’,, B;) instead of the uniform notation B (resp. B’) used up to now.

(3.20.1) For any point a = ( a l , a,) E E a i d arty r > 0, we hare B(a; r ) = Bl(al ; r ) x B,(a, ; r ) and B’(a; r ) = B;(a, ; r ) x B;(a, ; r ) . This follows at once from the definition of d.

(3.20.2) I f A , is an open set in El, A , an open set in E,, then A, x A, is open in E, x E, . For if a = (a,, a , ) E A l x A,, there exists rl > 0 and r, > 0 such that B,(a,; rl) c Al, B,(a,; r z ) c A,; take r = min(r,, r , ) ; then by (3.20.1), B(a; r ) c A, x A,,

(3.20.3) For any pair of sets A, c El, A, c E,, A, x A, = A , x A,; in particular, in order that A, x A , be closed in E, a necessary and suficient condition is that A, be closed in El and A, closed in Ez . If a = (a,, a,) EA, x A,, for any&> 0 there is, by assumption, an x1 E A , and an x, E A, such that d,(a,,x,)< E , d z ( a z ,x,) < E ; hence if x = (x,,x,),

20

PRODUCT OF TWO METRIC SPACES 73

d(a, x) < E . On the other hand, if (a,, a,) $A, x A, then either a, $ A, o r a, # A,; in the first case, the set (El - A,) x E, is open in E by (3.20.2), contains a and has a n empty intersection with A, x A,, hence a 4 A, x A,; the other case is treated similarly.

(3.20.4) Let z f ( z ) = (f,(z),f,(z)) be a mapping of a metric space F into E = El x E,; in order that f be continuous at a point z o , it is necessary and --f

suflcient that both f l and f 2 be continuous at zo . Let xo = ( f i ( z o ) f2(z0)); , then we have

by (3.20.1),and the result follows from (3.11 .I) and (3.6.3).

(3.20.5) Let f = ( , f l , f , ) be a mapping of a subspace A of a metric space F into El x E,, and let a EA; in order that f haoe a limit at the point a

with respect to A, a necessary and suflcient condition is that both limits b, = lirn f,(z), b , = lirn f2(z) exist, and then the limit of f is =-+a,z

z-a, z E A

b = (h,b2).

E

A

This follows at once from (3.20.4)and the definition of a limit. In particular:

(3.20.6) In order that a sequence of points z, = (x,,, y,) in E = El x E, be confiergent, a necessary and suflcient condition is that both limits a = lirn x,, b = lim y,, exist and then lim z, = ( a , b). n+

m

?I-+ m

n+ m

Note that for cluster values of sequences, if (a, b) is a cluster value of a is a cluster value of (x,,) and b a cluster value of (y,,), as follows from (3.20.6)and the definition of cluster values; but it may happen that ((x,, y,)) has no cluster value, although both (x,,) and (y,,) have one: for instance, in the plane R2, take x,, = l / n , y,, = n, x,,+~= n, y2n+l= l/n. However, if (x,) has a limit a, and b is a cluster value of(y,,), then (a, b) is a cluster value of ( ( x , , y,)), as follows from (3.20.6). ( ( x , , y,)),

74

I l l METRIC SPACES

(3.20.7) In order that a sequence of points z, = (x,, y,) in El x E, be a Cauchy sequence, a necessary and sufficient condition is that each of the sequences (x,), (y,) be a Cauchy sequence. This follows at once from the definition of the distance in El x E2 and the definition of a Cauchy sequence.

(3.20.8) Let z -+f(z) = ( f i ( t ) ,f2(z)) be a mapping of a metric space F into El x E,; in order that f be uniformly continuous, it is necessary and sufficient that both fland f2 be uniformly continuous. This follows immediately from the definitions.

(3.20.9) I f E is a metric space, d the distance on E, the mapping d of E x E into R is uniformly continuous. For Id(x, y ) - d(x’, y’)l

< d(x,x’) + d(y, y’) by the triangle inequality.

(3.20.10) Theprojectionspr, andpr,areuniformlycontinuous in E = El x Ez . Apply (3.20.8)to the identity mapping of E.

(3.20.11) For any a, E E2 (resp. a, E El), the mapping x1 +(xl, a,) (resp. x, 4(al, x,)) is an isometry of El (resp. E,) on the closed subspace El x {a,} (resp. {all x E2) of El x E, * This is an obvious consequence of the definition of the distance in El x E,, and of (3.20.3).

(3.20.12) For any open (resp. closed) set A in El x E, , and any point a , E El, rhe cross section A(al) = pr2(A n ({al} x E,)) is open (resp. closed) in E, .

By (3.20.11)it is enough to prove that the set A

n ({a,} x E,) is open

(resp. closed) in {al} x E z , which follows from (3.10.1)and (3.10.5).

(3.20.13) For any open set A in El x E,, pr, A (resp. pr, A) is open in El (resp. Ez).

20 PRODUCT OF TWO METRIC SPACES 75

Indeed, we can write pr, A =

u

XI

A(x,), and the result follows from

EEI

(3.20.12) and (3.5.2). Note that if A is closed in El x E,, pr, A needs not be closed in El. For instance, in the plane R2,the hyperbola of equation x y = 1 is a closed set, but its projections are both equal to the complement of (0) in R, which is

not closed. Let f be a mapping of E = El x E, into a metric space F. I f f is continuous at a point (al,a,) (resp. uniformly continuous), then the mapping f ( . , a,): x, --f f (x,, a,) is continuous at a, (resp. uniformly continuous). (3.20.14)

That mapping can be written x1 4(x,, a,) -+f ( x l ,a,), hence the result follows from (3.20.11), (3.11.5), and (3.11.9). The converse to (3.20.14) does not hold in general. A classical counterexample is the function f defined in R2 by f (x, y ) = xy/(x2 y 2 ) if ( x ,y ) # (0,O)andJ(0,O) = 0;f is not continuous at (0, 0), forf ( x ,x ) = 1/2 for x # 0.

+

Let El, E 2 , F,, F, befour metric spaces,f , (resp. f,)a mapping of El into F1(resp. of E, into F,). In order that the mappingf:(xL,x 2 )+ (f,(x,),f,(x,)) of El x E, into F, x F, be continuous at a point (a,,az) (resp. uniformly continuous), it is necessary and su$cient that f , be continuous at a, and f , at a, (resp. that both f, and f , be uniformly continuous). (3.20.15)

The mapping (x,, x,) +fi(xl) can be written f i 0 prl, hence the sufficiency of the conditions follow from (3.20.4), (3.20.8), and (3.20.10). On the other hand, the mapping fi can be written x, -+ pr,( f ( x , , a,)) and the necessity of the conditions follows from (3.20.14) and (3.20.10). (3.20.16) Let El, E, be two nonempty metric spaces. In order that E = El x E,

be a space of one of the following types: (i) discrete; (ii) bounded; (iii) separable; (iv) complete; (v) compact; (vi) precompact; (vii) locally compact; (viii) Connected; (ix) locally connected; it is necessary and suficient that both El and E2 be of the same type.

76

I l l METRIC SPACES

The necessity part of the proofs follows a general pattern for properties (i) to (vii): from (3.20.11) it follows that El and E, are isometric to closed subspaces of El x E,; and then we remark that properties (i) to (vii) are “inherited” by closed subspaces (obvious for (i) and (ii), and proved for properties (iii) to (vii) in (3.10.9), (3.14.5), (3.17.3), (3.17.4), (3.18.4)). For property (viii), the necessity follows from (3.19.7) applied to the projections pr, and pr,; similarly, if E is locally connected, for any (a,, a 2 )E E and any neighborhood V, of a, in El, V, x E, is a neighborhood of (a,, u2), hence contains a connected neighborhood W of (a,, a,); but then pr, W is a connected neighborhood of a, contained in V,, by (3.19.7) and (3.20.13). The suficiency of the condition for (i) and (ii) is an obvious consequence of the definition of the distance in E, x E, . For (iii), if D,, D, are at most denumerable and dense in E,, E, respectively, then D, x D, is at most denumerable by (1.9.3), and is dense in E by (3.20.3). For (iv), if (z,) is a Cauchy sequence in E, then (pr, z,) and (pr, z,) are Cauchy sequences in El and E, respectively by (3.20.7), hence they converge to a,, a2 respectively, and therefore (z,) converges to (a,, a*) by (3.20.6). For (vi), if (A,) (resp. (B,)) is a finite covering of El (resp. E2) by sets of diameter < c , then ( A i x B,) is a finite covering of El x E, by sets of diameter < E ; and by (3.16.1), the sufficiency of the condition for (iv) and (vi) proves it also for (v). The proof for (v) yields a proof for (vii) if one remembers the definition of neighborhoods in El x E,. For (viii), let (a,, a2), (b,, b 2 ) be any two points of E; by (3.20.11) and the assumption, the sets { a , } x E, and El x {b,} are connected and have a common point (a,, b,). Hence their union is connected by (3.19.3), and it contains both (a,, a,) and (b,, b 2 ) ;therefore, the connected component of (al, a 2 ) in E is E itself. The same argument proves the sufficiency of the condition for (ix), remembering the definition of the neighborhoods in E. (3.20.17) I n order that asubset A of El x E, be relatively conlpacr, a )iecessary and suficient condition is that prl A ai7d pr, A be relatioely compact ii7 El ai7d E, respectively. The necessity follows from (3.17.9) applied to pr, and pr,; the sufficiency follows from (3.20.16), (3.20.3) and (3.17.4).

All definitions and theorems discussed in this section are extended at once to a finite product of metric spaces.

20

PRODUCT OF TWO METRIC SPACES

77

PROBLEMS

1. Let E, F be two metric spaces, A a subset of E, B a subset of F ; show that fr(A x B) = (fr(A) x B) u (A x fr(B)). 2. Let E, F be two connected metric spaces, A # E a subset of E, B # F a subset of F; show that in E x F the complement of A x B is connected. 3. (a) Let E, F be two metric spaces, A (resp. B) a compact subset of E (resp. F). If W is any neighborhood of A x B in E Y F, show that there exists a neighborhood U of A in E and a neighborhood V of B in F such that U x V c W (consider first the case in which B is reduced to one point). (b) Let E be a compact metric space, F a metric space, A a closed subset of E x F. Show that the projection of A into F is a closed set (use (a) to prove the complement of prz A is open). (c) Conversely, let E be a metric space such that for every metric space F and every closed subset A of E x F, the projection of A into F is closed in F. Show that E is compact. (If not, there would exist in E a sequence (x,) without a cluster value. Take for F the subspace of R consisting of 0 and of the points l / n (n integer 2 1) and consider in E Y F the set of the points ( x n , lit?).) 4. Let E be a compact metric space, F a metric space, A a closed subset of E x F, B the (closed) projection of A into F. Let yo E B and let C be thecross section A-’(yo) = { x E E I ( x , y o ) E A]. Show that for any neighborhood V of C in E, there is a neighborhood W of yo in F such that the relation y c W implies A-’(y) C V (“continuity of the “roots” of an equation depending on a parameter”). (Use Problem 3(a).) 5. (a) Let f be a mapping of a metric space E into a metric space F, and let G be the graph off’in E x F. Show that iff is continuous, G is closed in E x F, and the restriction of pr, to G is a homeomorphism of G onto E. (b) Conversely, if F is compact and G is closed in E x F, then f is continuous (use Problem 3(b)). (c) Let F be a metric space such that for any metric space E, any mapping of E into F whose graph is closed in E x F is continuous. Show that F is compact (use the construction of Problem 3(c)). 6. Let E, F, G be three metric spaces, A a subset of E x F, B a subset of F x G, C = B 0 A = {(x, z ) E E x G j3y E F such that (x, y ) E A and (y, z) E B}. Suppose bothA and B are closed and the projection of A into F is relatively compact; showthat C is closed in E x G (use Problem 3(b)). 7. Let (En) ( n 2 I ) be an infinite sequence of nonempty metric spaces, and suppose that for each ti, the distance d,, on En is such that the diameter of E. is < 1 (see Section 3.14,

Problem 2(b)). Let E be the infinite product

PIXI

En. m

(a) Show that on E the function d((xn),( y o ) )=

“=I

d,(x., y.)/2” is a distance.

(b) For any x = (x.) E E, any integer m >, 1 and any number r > 0, let VJx; r ) be the set of all y = (y.) E E such that & ( x k , yk) < r for k < m. Show that the sets V,,(x;r ) (for all m and v) form a fundamental system of neighborhoods of x in E. (c) Let (x‘”’))be a sequence of points x(‘”)= (x,!“’)).~~ of E; in order that ( x c m )converge ) to a = (a,) in E (resp. be a Cauchy sequence in E), it is necessary and sufficient that for each 12 the sequence (x!””).,~I converge to a. in En (resp. be a Cauchy sequence in En). In order that E be a complete space, it is necessary and sufficient that each En be complete.

78

Ill

METRIC SPACES r

(d) For each n , let A, be a subset of E, ; show that the closure in E of A equal to

fi A,.

=

IT A,, is

"=I

(e) In order that E be preconipact (resp. compact), i t is necessary and sufficient that each En be preconipact (resp. compact). (f) In order that E be locally compact, i t is necessary and sufficient that each En be locally compact, and that all En, with the exception of a finite number at most, be compact. ( 8 ) In order that E be connected, i t is necessary and sufficient that each E. be connected. (11) In order that E be locally connected, it is necessary and sufficient that each E,, be locally connected and that all Em,with the exception of a finite number at most, be connected.

CHAPTER I V

ADDITIONAL PROPERTIES OF THE REAL LINE

Many of the properties of the real line have been mentioned in Chapter 111, in connection with the various topological notions developed in that chapter. The properties gathered under Chapter IV, most of which are elementary and classical, have no such direct connection, and are really those which give to the real line its unique status among more general spaces. The introduction of the logarithm and exponential functions has been made in a slightly unorthodox way, starting with the logarithm instead of the exponential; this has the technical advantage of making it unnecessary to define first amin(117, n integers > 0) as a separate stepping stone toward the definition of ax for any x. The Tietze-Urysohn theorem (Section 4.5) now occupies a very central position both in functional analysis and in algebraic topology. It can be considered as the first step in the study of the general problem of exfendirlg a continuous mapping of a closed subset A of a space E into a space F, to a continuous mapping of the whole space E into F; this general problem naturally leads to the most important and most actively studied questions of modern algebraic topology.

1. C O N T I N U I T Y OF ALGEBRAIC OPERATIONS

+ y of R x R into R is uniformly continuous. This follows at once from the inequality

(4.1.I) The mapping (x,y ) -+ x

+ < Ix’ - XI + ly’ - yl

I(x’ + y’) - ( x y)l and the definitions.

79

IV ADDITIONAL PROPERTIES OF THE REAL LINE

80

(4.1.2) The mapping (x, y ) -+ xy of R x R into R is continuous; for any a E R, the mapping x -+ a x of R into R is uniformly continuous. Continuity of xy at a point (xo, yo) follows from the identity XY - XOYO = Xo(Y

- Y o ) + (x - X 0 ) Y O

+ (x - X O ) ( Y

- Yo).

Given any E > 0, take 6 such that 0 < 6 < 1 and 6(1x01 + lyol + 1) < E ; then the relations Ix - xo( < 6, ly - yoJ < 6 imply [xy - ?coyo[< c. Uniform continuity of x -+ax is immediate, since [ax' - ax1 = J a J. Jx'- XI.

(4.1.3) Any continuous mapping f of R irito itsdf such that f(x +f ( y ) is of type x -+ cx, with c E R.

+ y) =

f(x)

Indeed, for each integer n > 0, we have, by induction on n, f ( n x ) = nf(x); on the other handf(0 + x) =f ( 0 ) + f ( x ) , hencef(0) = 0, and f ( x + (-x)) = f ( x ) f ( - x ) = f ( 0 ) = 0, hence , f ( - x ) = -f(s). From that it follows that for any integer n > 0, f ( x / n ) = f ( x ) / n , hence for any pair of integers p , q such that q > 0, f ( p x / q ) =!f'(x)/q; in other words, f ( r x ) = rf(x) for any rational number r. But any real number t is limit of a sequence (r,) of rational numbers (by (2.2.16) and (3.13.13)),hence, fromtheassumptionon,fand (4.1.2) f ( t x ) = f(lim r,x) = limf(r,x) = lim r,f'(.x) = f ( x ) . lim r, = t f ( x ) . Let then,

+

n-1 a0

n-tm

n-tm

c = . f ( l ) , and we obtainf(x) = c x for every

(4.1.4)

x E R.

n+

cc

The mapping x -+ l/x is continuous at ecerj, poitit xo # 0 in R.

For given any E > 0, take 6 > 0 such that 6 < min(lxol/2, ~Ix,[~/2); then the relation Jx-xoJ < 6 first implies 1x1 > Jxol- 6 > 1x01/2, and then Il/x - l/xol = 1x0 - xl/lxx,I Q 21x0 - xl/lxo[2< E .

Any rational function (xl, . . . , x,) -+ P(xl, . . . , x,)/Q(x,, . . . , x,) where Q are polynomials with real coeficients, is continuous at each point (al, . . . , a,,) qf R" where Q(a,, . . . , a,) # 0. (4.1.5)

P

and

The continuity of a monomial in R" is proved from (4.1.2) by induction on its degree, then the continuity of P and Q is proved from (4.1.1) by induction on their numbers of terms; the final result follows from (4.1.4).

1

(4.1.6) The inappings

continuous in R x R.

CONTINUITY OF ALGEBRAIC OPERATIONS

(x,y ) + sup(x, y ) and

81

(x, y ) --+ inf(x, y ) are unifornzly

+ +

Ix - y ( ) / 2 and inf(x, y ) = (x As sup(x, y ) = (x y the result follows from (4.1.I) and (3.20.9).

+ y - (x - y ( ) / 2 ,

(4.1.7) All open intervals in R are homeomorphic to R.

From (4.1 .I .) and (4.1.2) it follows that any linear function x -+ a x + b, with a # 0, is a homeomorphism of R onto itself, for the inverse mapping x - i a - ' x - a-'b has the same form. Any two bounded open intervals ]LY, /?[, ] y , 6[ are images of one another by a mapping x --+ax+ b, hence are homeomorphic. Consider now the mapping x --t xi( I + 1x1) of R onto 1- 1, I[; the inverse mapping is x xi( 1 - 1x1) and both are continuous, since x --t 1x1 is. This proves R is homeomorphic to any bounded open interval; finally, under the preceding homeomorphism of R onto ] - I , + 1[, any unbounded open interval ]a, + a[or ] - co, a[ of R is mapped onto a bounded open interval contained in ] - I , + 1 [, hence these intervals are also homeomorphic to R .

+

--f

+

(4.1.8) With respect to R x R, the ,fuiiction (x, y ) x y has a lilllit at euerypoint (a, b) g f R x R, except at the points (- GO, + co) and ( + co, - 00); that litnit is equal to + cc (resp. - 00) if' one at least of tl7e coordinates a, b is +co (resp. -m). -+

Let us prove for instance that if a # - co, x + y has a limit equal to +co at the point (a, + co). Given c E R, the relations x > b, y > c - b imply x + y > c, and the intervals ]b, + co] and ]c - 6, + co] are respectively

neighborhoods of a and + GO if b is taken finite and < a ; hence our assertion. The other cases are treated similarly.

With respect to R x R, the function (x, y ) xy has a limit at every point (a, 6 ) qf R x 8, except at the points (0, +a),(0, -a),( f a , 0), (- co,0 ) ;that litnit is equal to + co (resp. - 00) if one at least of the coordinates a, b is injnite, and ifthey have the same sigii (resp. opposite signs). (4.1.9)

-+

Let us show for instance that if a > 0, xy has the limit + co at the point (a, co). Given c E R, the relations x > b, y > c/b, for b > 0, imply xy > c, and the intervals ]b, + co] and ]c/b, + GO] are neighborhoods of a and + 00, if b is taken finite and
+

IV ADDITIONAL PROPERTIES OF THE REAL LINE

82

We omit the proofs of the following two properties: (4.1.lo)

lim l/x = 0, X-rfU3

lim l/x =

$00,

x-to. x > 0

lim

l/x = -co.

x+o,x
(4.1 .I 1) The mappings (x, y ) + sup(x, y ) and (x, y ) + inf(x, y ) are continuous in R x B.

2. MONOTONE FUNCTIONS

Let E be a nonempty subset of the extended real line R. A mapping f of E into is called increasing (resp. strictly increasing, decreasing, strictly decreasing) if the relation x < y (in E) implies f(x) f ( y ) ) ;a function which is either increasing or decreasing (resp. either strictly increasing or strictly decreasing) is called monotone (resp. strictly monotone); a strictly monotone mapping is injective. If f is increasing (resp. strictly increasing), -f is decreasing (resp. strictly decreasing). If ft g are increasing, and f g is defined, f + g is increasing; if in addition f and g are both finite and one of them is strictly increasing, then f g is strictly increasing.

w

+

+

w,

(4.2.1) Let E be a nonempty subset of and a = sup E; i f a # E, then, for any monotone mapping f of E into R, lirn f ( x ) exists and is equal to x-ta,xsE

sup f ( x ) i f f is increasing, to inf f ( x ) i f f is decreasing. (Theorem of the xsE

monotone limit.)

XEE

Suppose for instance f is increasing, and let c = supf (x). If c = - 00, XEE

f is constant (equal to - 00) in E and the result is trivial; if c > - co, for any b < c, thereisx E Esuchthatb
2 MONOTONE FUNCTIONS

83

f ( c ) # f ( a ) , and if we had for instance f(c) > f ( b ) , there would then be an x such that a < x < c and f(x)=f(x)= f ( b ) by Bolzano's theorem(3.19.8), contrary to our assumption. Similarly one sees that f (c)
PROBLEMS

+

1. Let f be a mapping of

+

R into R such that f ( x y ) =f ( x ) f ( y ) . Show that if, in an interval ]a, b[, f is majorized, then f is also minorized in ]a, b[ (if c is a fixed point in the interval ]a, b[, consider pairs of points x , y in that interval such that x < c < y and (y - c)/(c - x ) is rational). Under the same assumption,fis bounded in any compact interval, and continuous in R, hence of the form f ( x ) = cx (same method). (It can be proved, using the axiom of choice, that there exist solutions of f ( x y ) = f ( x ) +f(y) which are unbounded in every interval.) 2. Let b be an integer > 1. (a) Show that for any infinite sequence (c,) of integers such that 0 < c,, < b - I ,

+

the series

c c,/b" converges to a number m

n=0

exists a sequence (c.) such that 0 < c,

x

E

[0, 11. Conversely, for any x

E

[O, 1 1 there

c m

< b - 1 for every n and x = " = O c,/b"; that sequence

is unique if x has not the form k/bm(k and m natural integers); otherwise, there are exactly two sequences (c,) having the required properties. (Use the fact that for any integer m,> 0, and any X E [0, I], there is a unique integer k such that k/b" < x < (k l)/b".) (b) Using the case b = 2 of (a), and Problem 5 of Section 1.9, show that [0, I ] (hence R itself, see (4.1.7)) is equipotent to the set %(N).

+

(c) Let K be the subset of [0, 1J consisting of all numbers of the form

m

c,/3", with

"=O

c, = 0 or c, = 2 ("triadic Cantor set "). Show that K is compact; its complement in [0, 11 is a denumerable union of open nonoverlapping intervals (3.19.6) ; describe these intervals, and show that the (infinite) sum of their lengths is 1.

(d) For each x

E

K, with x =

m

"=O

c,/3", let f ( x ) be the real number

m

1b./2", where

"=O

IV ADDITIONAL PROPERTIES OF THE REAL LINE

84

m

b.

= c,/2

(when x can be written in two different ways as

corresponding numbers

"=a

4 3 " , show that the two

c,

h,,/2"are equal). Prove thatfisasurjectivecontinuous mapping

n=o

3.

of K onto the interval [0, I ] of R , and show that K and R are equipotent. Furthermore it is possible to extendf'to a continuous mapping of 1 = [0, I ] onto itself, which is cnn.strrnt in each of the connected components (3.19.6) of I - K. (a) Let E be a metric space satisfying the following condition: for each finite sequence s = (EJ, whose terms are equal to 0 or I , there is a nonempty subset A, such that: (i) E is the union of the two subsets A(,,, A ( ] ) ,and for each finite sequence s of i ! terms, if s', s'' are the two sequences of I I I terms whose first I I terms are those of s , A, A,. U A,,,; (ii) for each infinite sequence ( E , , ) , , ~ whose terms are equal to 0 or I , if s, = ( F ~ ) I <,, , the diameter of A,, tends to 0 when I I tends to - 1 T ~ , and the intersection of the A,, is not empty. Under these conditions, show that there exists a continuous mapping of the triadic Cantor set K (Problem 2) oilto E, and in particular E is compact. (b) Conversely, let E be a n arbitrary compact metric space. Show that there exists a continuous mapping of K onto E. (Apply the method of (a), and the definition of precompact spaces (Section 3.16); observe that properties (i) and (ii) d o not imply that the two sets A,, and A,.. need be different from A, for all sequences s.) (c) If in addition E is totally disconnected, and has no isolated points (3.10.10), then E is horneomo~phic to K . (First prove that for every E > 0 there is a covering of E by a finite number of sets Ai which are both open and closed and have a diameter < E ; to that purpose use Problem 9(a) of Section 3.19. Then apply the method of (a).) (a) Let E (resp. F) be the set of even (resp. odd) natural integers; if, to each subset X of N, one associates the pair (X n E, X n F), show that one defines a bijection of T(N) onto V(E) x b(F). (b) Deduce from (a) and from Problem 2(b) that R" and R are equipotent for all I Z> 1 (but see Section 5.1, Problem 6). Let I be the interval [0, I ] in R . Show that there exists a co/iti/irioris mapping f of 1 otim the "square" I x I (a "Peano curve"). (First show that there is a continuous mapping of the Cantor set K onto I x 1 (Problem 3), and then extend the mapping by linearity to the connected components of the complement of K in 1.) Let y be a mapping of the interval 10, I ] into the interval [ - - I , I], and suppose that lim g ( s ) 0. Show that there exist a continuous decreasing mapping g 1 and a

+

4.

5.

6.

7

I-0, x > o

0, continuous increasing mapping y 2 of [O, I ] into [ - I , I], such that gl(0) = g,(O) and g,(.u) < g(x) < gz(.u) for 0 c: x < I. (For each integer 11, consider the g.1.b. x,, of the set of points x such that y(x) 2 I j ~ r . ) ~~

3. L O G A R I T H M S A N D E X P O N E N T I A L S

(4.3.1) For any number a > 1, there is a utiique increasilig niappirig f of RT = 10, +a[illto R sirch t l i a t f ( s y ) = f ( s ) + f ( y ) a n d f ( a ) = I ; nioreowr, f is a lioriieoniorphistiz of RT onto R.

3

85

LOGARITHMS A N D EXPONENTIALS

We first prove a lemma: (4.3.1 . I ) For any x > 0, there is an integer m (positice or negatioe) such tlzat am< x < am+ I . a"

Suppose first s 2 1. The sequence (a") is strictly increasing. If we had lim a" = sup a"

< x for all integers n > 0, then (by (4.2.1) and (3.15.4)) b =

n

n-t m

would be finite, > 1 and
7,

=a

.lim a" n+m

by (4.1.2), hence b = ah, which contradicts the assumption a > I . Therefore there is a n integer n such that x > a"; take 111 + 1 as the smallest of these integers. If on the contrary 0 < s < I , then s-l > 1. and if a"' < s-'< a " + ' , we have a - ( " f le ) x< a - ( " ' + l ) + l (2.2.13). Suppose there exists a functionJ'having the properties listed in (4.3.1); then ,/is a homomorphism of the multiplicative group RT into the additive group R, and therefore we must havef(1) = O,,/( x") = n . f ( x ) for any x > 0 and any integer I I (positive or negative), and in particular f( a " ) = n. Moreover, if am < x" < a"", we must have f ( a ' " ) < f ( s " ) < f ( a " + ' ), in other words 117 < n ..f'(x) < ni + I , hence nijn <,/(x) and If(x) - rnjnl < I j n . This shows that if we denote by A, the set of rational numbers mjn ( n positive ~ or negative, n 3 I ) such that a"' 6 s", (note that an'< x" and a"" 6 xnq, where q is an integer > O , are equivalent relations (2.2.13)), we must have f ( x ) = sup A,, which shows ,/ is unique. To prove the existence o f f , it remains to prove that the mapping f: x + sup A, verifies all our conditions. Let x and y be any two elements of R T ; for any integer n 2 I , let m, YYI' be such that am < x"< am+' and am' < y~~ 6 a m ' + 1 ., from these relationsitfollows that a""" < (xy)" hence we have In m+l nz ' Mt 1 - I1 n n

+

I??

+ lnl n


<

in

+ ni' + 2 n

9

and also m

+ M' n

GA x )

+ f(Y)<

m+m'+2

We conclude that

I, f ( S Y )-f ( x ) -f ( Y >I < 2 1 / 7 9 and as n is arbitrary, f ( V )=f(x>+ f(v>.

86

IV ADDITIONAL PROPERTIES OF THE REAL LINE

From (4.3.1-1) it follows that for any z > 1 , there is an integer n 3 1 such that a 0; from which it follows that f is strictly increasing, since if x < y , then y = zx with z > 1 andf(y) = f ( x ) + f ( z ) > f ( x ) . On the other hand, we have the following lemma: (4.3.1.2) For any integer n 2 1, there is a z > I such that Z" 6 a.

Remark that there is an x such that 1 < x < a, hence a = xy with y > I ; if z1 = min(x, y ) , we have z: 6 x y = a and z1 > 1. By induction define z, > I such that z: < z , , - ~ ,hence z:" 6 a, and a fortiori z l < a. The lemma shows that 0
-< z

and

X

x-6

1 z

-> - : x

then I f ( y ) -f(x)I < f ( z ) 6 l / n for ly - XI <6, which proves f is continuous. By (4.2.2),f is thus a homeomorphism of R*, onto an interval I of R ; but that interval is necessarily R itself, sincef ( a") = n is an arbitrary integer. (4.3.2) For any number a > 0 and # 1, there exists one and only one continuous rnappingfofRz into R such t h a t f ( x y ) = f ( x ) + f ( y ) andf(a) = 1 .

Let b > 1 ; from (4.3.1) we have a homeomorphism f o of RT onto R such that f o ( x y ) = f o ( x ) f o ( y ) and fo(b) = 1 ; let go be the inverse homeomorphism, such that go(x y ) = go(x)go(y)and g o ( l ) = b. I f f verifies the conditions of (4.3.2), then h =f go is a continuous mapping of R into itself, such that h(x + y ) = h(x) + h(y); by (4.1.3), we have h(x) = cx, and therefore f ( x ) = cfo(x), and there is only one value of c for which f ( a ) = 1, namely c = l/jo(a) (as a # 1, we havefo(a) # 0 =fo(l)).

+

+

The mapping characterized in (4.3.2) is called the logarithm of base a, andf(x) is written log, x . From the proof of (4.3.2) it follows at once that if a, b are > O and # 1 , log, x and lo& x are proportional, and making x = a yields (4.3* 3)

log,, x = log, a log, x.

From (4.3.1) and (4.2.1) it follows that if a > 1, lim log,x

= -00,

x+o

lim l o g , x = +a;if a < 1,lim l o g a x = +a, lim log,x= -a. For X-'+OO

r-0

x-+m

any a > 0 and # 1 , the inverse mapping of x 4 log, x is called the exponential of base a and written x + a X (which is a coherent notation, since log,(a") = n, and therefore for integral values of x , the new notation

3 LOGARITHMS AND EXPONENTIALS

87

has the same meaning as the algebraic one). In addition, we define 1" to be 1 for all real numbers x . Then, for a > 0, x , y arbitrary real numbers, = d a y , a-" = l / d , a' = 1. Replacing x by b" we have by definition in (4.3.3) yields (b > 0, x real)

log,(b") = x log, b

(4.3.4)

and replacing b by ay in that formula gives ( x , y real, a > 0).

(ax)Y= axy

(4.3.5)

For a > 1, x lim ax = x++m

-+

ax is

+ co; for

strictly increasing and such that lim ax = 0,

a < I, x

x--m

-+

ax is strictly decreasing, and such that

lirn ax = +a,lim ax = 0.

x4-m

x4-m

(4.3.6) The mapping (x, y ) + x Y is continuous in R*, x R, and tends to a limit at each point of i? x i? in the closure of R: x R and distinct from (0, 0), (+a, 01, (1, +a), (1, - 0 3 ) .

From (4.3.4), we have xy = a Y " O g a " (a fixed number >l), hence the result by (4.1.2) and (4.1.9). (4.3.7) Any continuous mapping g ofR*, into itselfsuch that g(xy) = g(x)g(y) has the form x -+ x", with a real.

+

Indeed, if b > 1, f ( x ) = log, g(b") is such that f ( x y ) =f ( x ) + f ( y ) , for real x , y, and is continuous, hence f ( x ) = c ax by (4.1.3), therefore g(b") = 6'" = (b")', which proves the result. As log,(x") = a * log,, x, we see that if a > 0, x X" is strictly increasing, and strictly decreasing if a < 0; moreover if a > 0, lim X" = 0, ~~

lim x" = +co; if X-++OZ

~~

a < 0, lim

X"

=

+a,lim x" = 0.

"40

r-0 .. .

For

a # 0,

X++W2

X + X " is therefore a homeomorphism of R*, onto itself, by (4.2.2); the inverse homeomorphism is x -+ XI'".

PROBLEM

+

Letfbe a mapping of R into itself such thatf(x y ) ==f(x) +f(y)andf(xy) =f(x)f(y). Show that either f(x) = 0 for every x E R, or f(x) = x for every x E R. (If f(1) # 0, then f(1) = 1 ;in the second case, show thatf(x) = x for rational x, and using the fact that every real number I > 0 is a square, show that f is strictly increasing.)

88

IV ADDITIONAL PROPERTIES OF THE REAL LINE

4. COMPLEX N U M B E R S

We define two mappings of the set R2 x R2 into R2 by ( ( x ,Y ) , ( X I , I?’)) ((X,

JI),

( X I , J]’))

-+

-+

( x + x’,y (XX’

+ Y’)

- J’J”,

XJ”

+ YX’).

They are called respectively addition and multiplication, and written

(z, z‘) -+ z + z’ and (z, z’)

+ zz’.

For these two mappings, axioms (I) (Section 2.1) of a field are satisfied, by taking 0 = (0, 0), 1 = (1,0), and

if z = (x, y ) # 0 (which, by (2.2.8) and (2.2.12), implies x 2 + y 2 # 0). The field thus defined is written C and called the .field of complex nunzbers, its elements being called complex numbers. The mapping x (x, 0 ) of R into C is injective and preserves addition and multiplication, hence we identify R with the subfield of C consisting of the elements ( x , 0).The element i = (0, 1) is such that i 2 = (- 1,O) = - 1, and we can write ( x , y ) = x + iy for any (x, y ) E C ; if z = x + iy, x , y being real, x is written .!Bz and called the real part of z, J) is written 9 z and called the iniaginarj~partof z. -f

(4.4.1) Any ratiomlfunctiori (zl, . . . , z,) -+ P(zl, . . . , z,)/Q(z,, . . . , z,) where P arid Q are polynomials with cornplex coeflcients, is continuous at each poirit ( a l , . . . , a,) of C“ such that Q(al, . . . , a,) # 0.

This is proved as (4.1.5) by using the analogs of (4.1.1), (4.1.2) and (4.1.4), which follow at once from the formulas given above for sum, product and inverse of complex numbers, and from (3.20.4) and (4.1.5). For any complex number z = x iy, the number Z = x - ijj is called

+

+

-

the conjugate of z. We have Z = z, z + z’ = Z Z’, zz’ = Z .Z’, in other words z + f is an automorphism of the field C, which is bicontinuous by (3.20.4) and (4.1.2); real numbers are characterized by 2 = z, numbers of the form ix (x real, also called purely inm~inarpnumbers) by Z = - 2 . We have ZZ = x 2 + y 2 3 0 if z = x -t- i y ; the positive real number IzI = ( ~ 2 ) ” ~ is called the absolure value of z, and coincides with the absolute value defined in Section 2.2 when zis real. The relation Iz( = 0 is equivalent to z = 0. We have (zz’J2= zz’zz‘ = zZz’Z’ = I Z ( ~ ( Z ’hence ( ~ , lzz‘l = J z J* Jz’J, from which it follows that if z # 0, l l / z l = l / l z l . Finally, by direct computation, we check the triangle inequality -

I2

+ Z’I < I Z I + lz’l

89

5 THE TIETZE-URYSOHN EXTENSION THEOREM

which shows that ( z - z ’ [ = d(z, z’) is a distance defined on C = R x R, which is uniformly equivalent to the distance considered in Section 3.20. The balls for that distance are called discs. Any complex number z # 0 can be written in one and only one way as a product r i , with r > 0 and Ill = 1, namely by taking r = IzI and [ = z/lzI.

PROBLEM

+

Let f be a continuous mapping of C into itself such that f(z z’)=f(z) +f(z’) and f(zz’)=f(z)f(z’). Show that either f(z) = 0 for every z E C, or f is one of the mappings z + z,z + f (use (4.1.3)). (It can be proved, using the axiom of choice, that there are injective, nonsurjective and noncontinuous mappings f of C into itself, such that f(z z’)= f ( z ) +f(z’) and f(zz’) = f ( z ) f ( z ’ ) . Compare to the problem in Section 4.2.)

+

5. THE TIETZE-URYSOHN EXTENSION THEOREM

(4.5.1) (Tietze-Urysohn extension theorem) Let E be a metric space, A a closed subset of E, f a continuous bounded mapping of A into R. Then there exists a continuous mapping g of E into R which coincides with f in A and is such that sup g ( x ) = supf(y), XEA

YEA

inf g(x) = inf f (y). XEE

YEA

We may suppose that inf f ( y ) = 1, sup f ( y ) = 2 by replacing eventually

f by a mapping y --t c1f(y) + j?, c1 # 0 (the case in which f is constant is trivial). Define g ( x ) as equal tof(x) for x E A, and given by the formula YEA

YEA

g(4 = (inf ( f ( Y ) d(X9 Y)))/d(X,A) YEA

for x E E - A. From the inequalities 1 < f ( y ) < 2 for y E A and the definition of d(x, A), it follows that 1 d g ( x ) < 2 for x E E - A. We need therefore only prove the continuity of g at every point x E E. If x E A, the continuity follows from the assumption on f . In the open set E - A, we can write g(x) = h(x)/d(x,A) with h ( x ) = inf (f( y ) d(x, y ) ) , and as d(x, A ) is continuous YEA

and # O (by (3.8.9) and (3.11.8)), all we have to prove then (by (4.1.2)and (4.1.4)) is that h is continuous at every x E E - A. Let r = d ( x , A); for d(x, x’) d E < r , we have d(x, y ) < d(x‘,y ) + E , hence h(x) < h(x’) 2~ (since f ( y ) < 2), and similarly h(x’) ,< h(x) + 2 ~ which , proves the continuity of h. Finally, let us suppose x is a frontier point of A ; given E > 0, let r > 0

+

90

I V ADDITIONAL PROPERTIES OF THE REAL LINE

be such that for y E A n B(x; r ) , I f ( y ) -f(x)I < E . Let C = A n B(x; r ) , D = A - C; if x‘ E E - A and d(x, x‘) Q r / 4 , we have, for each Y E D, d(x’, y ) > d(x, y ) - d(x, x’) 2 3r/4, hence inf ( f ( y )4 x ’ , Y ) ) 2 3 4 4 ;

YED

< 2d(x’, x) d r / 2 , and therefore inf ( f ( Y ) 4x‘, Y ) ) = inf f(v>4 x ’ , Y)). Y E A YEC

on the other hand, .f(x) d(x’, x)

But, as f ( x ) - E we have


for y

E

C, and inf d(x’, y ) = d(x’,A), YEC

which proves that (g(x’)-f(x)I < E for x’ E E - A and d(x, x’) Q r / 4 ; on the other hand, ifx’ E A and d(x, x’) < r/4, Ig(x’) -f(x)I = If(x’)-f(x)I d E , and this ends the proof.

(4.5.2) Let A, B be two nonempty closed sets in a metric space E, such that A n B = @. Then there is a continuous function f deJined in E, with values in [0, I], such thatf(x) = 1 in A and,f(x) = 0 in B. Apply (4.5.1) to the mapping of A u B in R, equal to 0 in B and to 1 in A, which is continuous in A u B.

PROBLEMS

1. In a metric space E, let (F.) be a sequence of closed sets, A the union of the F, ; if x $ A, show that there exists a bounded continuous function f > 0 defined in E, such that f ( x ) = 0 and f(y) > 0 for each y E A (use (4.5.2) and (7.2.1)). 2. (a) Let E be a metric space such that every bounded set in E is relatively compact; show that E is locally compact and separable (use (3.16.2)). (b) Conversely, let E be a locally compact, noncompact separable metric space, d the distance on E; let (U,) be a sequence of relatively compact open subsets of E such that 0.t U., and E is the union of the sequence (U.) (3.18.3). Show that there exists a continuous real-valued function f i n E such that f ( x ) 6 n for x E n,, and f ( x ) 2 n for xEE(use (4.5.2)); the distance d‘(x, y ) = d(x, y ) i f ( x ) -f(y)j is then topologically equivalent to d , and for d’, any bounded set is relatively compact.

u,,

+

CHAPTER V

NORMED SPACES

The language described in Chapter I11 corresponded to that part of our geometric intuition covering the notions which intuitively remain unaltered by “deformations”; here we get much closer to classical geometry, as lines, planes, etc. are studied from the topological point of view (we recall that the purely algebraic aspects of these notions constitute linear algebra, with which we assume the reader is familiar). It is in this context that the notion of series gets its natural definition ; we have particularly emphasized the fact that for the most important type of convergent series (Section 5.3), the usual rules of commutativity and associativity of finite sums are still valid, which naturally leads to the conclusion that in that case, the ordering of the terms is completely irrelevant. This, for instance, enables one to formulate in a reasonable way the theorem on the product of two such series of real numbers (see (5.5.3)), in contrast to the nonsensical so-called “ Cauchy multiplication ” still taught in some textbooks, and which has no meaning for series other than power series of one variable. The fundamental results of this chapter are the continuity criterion (5.5.1), and F. Riesz’s theorem characterizing finite dimensional spaces (5.9.4), which is the key to the elementary spectral theory developed in Chapter XI. Of course, this chapter is only an introduction t o the general theory of Banach spaces and linear topological spaces, which will be further developed in Chapter XII, and which is fundamental for modern functional analysis. 1. N O R M E D SPACES A N D B A N A C H SPACES

In this and the following chapters, when we speak of a vector space, we always mean a vector space (of finite or infinite dimension) over the field 91

92

V

NORMED SPACES

of real numbers or over the field of complex numbers (such a space being respectively called real and complex vector space); when the field of scalars is not specified, it is understood that the definitions and results are valid in both cases.* When several vector spaces intervene in the same statement, it is understood (unless the contrary is specified) that they have the same field of scalars. A complex vector space E can also be considered as a real vector space by restricting the scalars to R ; when it is necessary to make the distinction, we say that this real vector space E, is underlying the complex vector space E; if E has finite dimension n over C,E, has dimension 2n over R.

A norm in a vector space E is a mapping (usually written x -+ \lxll, with eventual indices to the 11. .ll) of E into the set R of real numbers, having the following properties: (I)

I(x/I2 0 for every x E E.

(11) The relation jlxll = 0 is equivalent to x = 0. (111)

lllxll = 1 1 1* IIxII for any x E E and any scalar A.

(IV) Ilx inequality ”).

+ yll < IIxIl + llyll

for any pair of elements of E (“triangle

(5.1 . I ) If x -+ llxll is a norm on the vector space E, then d(x, y ) = (/x - yll is a distance on E such that d(x z , y z ) = d ( x , y ) and d(Ax, Ay) = lAl d(x, y ) f o r any scalar A.

+

+

The verification of the axioms of Section 3.1 is trivial. A normed space is a vector space E with a given norm on E; such a space is always considered as a metric space for the distance IIx - yl/. A Banach space is a normed space which is complete. If E is a complex normed vector space, x -+ llxll is also a norm on the underlying real vector space E, , and the metric spaces E and E, are identical ; hence if E is a Banach space, so is E, .

Examples of Norms The examples given in (3.2.1), (3.2.2), (3.2.3), and (3.2.4) are real vector spaces, and the distances introduced in those examples are deduced from norms by the process of (5.1.1). The normed spaces thus defined in examples (3.2.1) to (3.2.3) are complete by (3.20.16) and (3.14.3), hence (5.1.2)

* The product of a scalar and a vector x is indifferently written Ax or x x ; 0 is the neutral element of the additive group of the vector space.

I

93

NORMED SPACES A N D BANACH SPACES

Banach spaces. Example (3.2.4) will be the object of a special study in Chapter VII, and we shall see it is also a Banach space. (5.1.3) Examples corresponding to the preceding ones are obtained by replacing everywhere real numbers by complex numbers (and in example (3.2.2), squares ( x i - y,)' by / x i- yi12).

(5.1.4) Let I = [a, b] be a closed bounded interval in R, and E = gR(I) the set of all real-valued continuous functions in I ; E is a vector space ( f + g and Af being respectively the mappings t + f ( t ) g ( t ) and t + A f ( t ) ) . If we write

+

I l f II I

= jblf(t)ld 4

l l f l l , is a norm on E. The only axiom which is not trivially verified is (11), which follows from the mean value theorem (see Chapter VIII). It can be proved that E is not complete (see Problem 1). For other important examples of norms, see Section 5.7 and Chapter VI.

+

(5.1.5) !fE is a real (resp. complex) normed space, the mapping (x, y ) + x y is uniformly continuous in E x E; the mapping ( A , x ) + A x is continuous in R x E (resp. C x E); the mapping x -+Ax is uniformly continuous in E. The proofs follow the same pattern as those of (4.1.1) and (4.1.2); to prove for instance the continuity of (A, x) + Ax at a point ( A o , xo), we use the formula IIAx - Aoxoll = IIAo(x- xo) (A - Ao)xo (2 - Ao)(x- xo)II < 1101 . IIX - XOII + I2 - A01 * IIXOII + IA - 1 0 1 llx - xoll.

+

+

As a corollary of (5.1.5), it follows that any translation x + a + x and any homothetic mapping x + Ax ( A # 0) is a homeomorphism of E onto itself, for the inverse mapping is again a translation (resp. a homothetic mapping).

PROBLEMS

1. Let I = [0, I], and let E be the normed space defined in (5.1.4). (a) For any n > 3, let f . be the continuous function defined in I, such that f n ( t ) = 1 for o ~ r r l / 2 , f . ( r ) = O f o r1 / 2 + I / n < t S 1 , a n d t h a t f . ( t ) h a s t h e f o r m c r n t f / 3 , , i n t h e interval [1/2, 1/2 l / n ] (with constants cr. and to be determined). Show that in E,

+

Pn

94

V

NORMED SPACES

(fn)is a Cauchy sequence which does not converge (if there existed a limit g of (fn) in E, show that one would necessarily have g ( t ) = 1 for 0 < f C 3 and g ( t ) = 0 for < t < 1, which would violate the continuity of 9 ) . (b) Show that the distance on E defined in (5.1.4) is not topologically equivalent to the distance defined in (3.2.4). (Give an example of a sequence in E which tends to 0 for ] ] f - g l l l , but has no limit for the distance defined in (3.2.4).) 2. If A, B are two subsets of a normed space E, we denote by A B the set of all sums a + b, where a E A, b E B. (a) Show that if one of the sets A, B is open, A B is open. (b) Show that if both A and B are compact, A B is compact (use (3.17.9) and

+

+

(3.20.16)).

+

+

(c) Show that if A is compact and B is closed, then A B is closed. (d) Give an example of two closed subsets A , B of R such that A B is not closed (cf. the example given before (3.4.1)).

+

3. Let E be a normed space. (a) Show that in E the closure of an open ball is the closed ball of same center and same radius, the interior of a closed ball is the open ball of same center and same radius, and the frontier of an open ball (or of a closed ball) is the sphere of same center and same radius (compare to Section 3.8, Problem 4). (b) Show that the open ball B(0; r ) is homeomorphic to E (consider the mapping x+rxl(l lixll>). 4. In a normed space E, a segment is the image of the interval [0, I ] of R by the continuous mapping t + f a (1 - t)b, where a E E and b E E; a and b are called the extremities of the segment. A segment is compact and connected. A broken line in E is a subset L of E such that there exists a finite sequence of points of E having the property that if S, is the segment of extremities x1and x i + , for 0 < i < n - 1, L is the union of the S, ; the sequence ( x i )is said to define the broken line L (a given broken line may be defined in general by infinitely many finite sequences). If A is a subset of E, a, b two points of A , one says that a and b are linked by a broken line in A, if there is a sequence ( x ~ such that a = xo, b = x, and that the broken line L defined by that sequence is contained in A. If any two points of A can be linked by a broken line in A, A is connected. Conversely, if A C E is a connected open set, show that any two points of A can be linked by a broken line in A (prove that the set of points y E A which can be linked to a given point a E A by a broken line in A is both open and closed in A). 5. In a real vector space E, a linear oariety V is a set of the form a M, where M is a linear subspace of E; the dimension (resp. codimension) of V is by definition the dimension (resp. codimension) of M. If b $ V and if V has finite dimension p (resp. finite codimension q), the smallest linear variety W containing both b and V has finite dimension p 1 (resp. finite codimension q - 1). Let A be an open connected subset of a real normed space E, and let (V,) be a denumerable sequence of linear varieties in E, each of which has codimension 2 ; show that if B is the union of the V., A n (E - B) is connected. (Hint: Use Problem 4; if L is a broken line linking two points a , b of A n (E - B) in A, prove that there exists another broken line L’ “close” to L, contained in A n (E - B). To do that, observe that if x E E - B, the set of points y E E such that the segment of extremities x , y does not meet any V., is dense in E, using (2.2.17).) In particular, if the dimension of E is 3 2 , and if D is a denumerable subset of E, A n (E - D) is connected.

+

+

+

+

)

~

~

2

SERIES IN A NORMED S P A C E

95

6. If E is a real normed space of dimension 2 2 , show that a n open nonempty subset of E cannot be homeomorphic to any subset of R (use Problem 5). 7. (a) Show that in a normed space E, a ball cannot contain a linear variety (Problem 5 ) of dimension >O. (b) Let (En) be an infinite sequence of normed spaces having dimension > O ; show that

in the metric space E =

n E n , there is no norm such that the distance m I

llx-y/l is

"=O

topologically equivalent to the distance defined in Problem 7 of Section 3.20 (where d, is taken as a bounded distance on E, equivalent to the distance defined on Enby the norm on that space). (Use (a).)

2. SERIES IN A N O R M E D SPACE

Let E be a normed space. A pair of sequences ( x , ) , > , ~(s,),~ , is called a series if the elements x, ,s, are linked by the relations s, = xo x1 * . x, for any n, or, what is equivalent, by xo = s o , x, = s, - snV1 for n b 1 ; x, is called the nth term and s, the nth partial sum of the series; the series will often be called the series of general term x,, or simply the series (x,)

+ +

(and even sometimes, by abuse of language, the series is said to converge to s if lim s, n-+ m

and written s = xo + ...

= s; s is

+ x, + . * . or

+

m

1x,).

The series

n=O

then called the sum of the series m

s=

1x,; r, = s - s,

is called the

n=O

nth remainder of the series; it is the sum of the series having as kth term x , + ~ ;by definition lim r, = 0. n+ m

(Cauchy's criterion) Ifthe series ofgeneral term x, is convergent, E > 0 there is an integer no such that, f o r n 3 no and p k 0, IIs,+,, - s,II = I I X , + ~ + X , , + ~ I I < E . Conversely, i f that condition is satis-ed and if the space E is complete, then the series of general term x, is convergent.

(5.2.1)

then f o r any

+

This is merely the application of Cauchy's criterion to the sequence (s,) (see Section 3.14). As an obvious consequence of (5.2.1)it follows that if the series (x,) is convergent, lim x, = lim (s, - s , , - ~ )= 0; but that necessary condition n-t m

n-+ m

is by no means sufficient.

96

V

NORMED SPACES

(5.2.2) I f the series (x,) and are convergent and have sunis s, s', then the series (x, + x@ converges to the siini s + s' and the series (Ax,) to the sum As f o r any scalar A. (XI,)

Follows at once from the definition and from (5.1.5).

(5.2.3) If (x,) and (x;) are ~ M ' Oseries such that xi = x, except f o r a finite number of indices, they are both concergent or both nonconcergent. For the series (x; - x,) is convergent, since all its terms are 0 except for a finite number of indices.

(5.2.4) Let (k,) be a strictly increasing sequence of integers 3 0 with ko = 0 ;

1 xp, then the series (y,) conrerges

kn+i-l

ifthe series (x,) coilverges to s , and ij-y, =

p=k,

also to s. This follows at once from the relation

n

k,, + I - I

i=O

j=O

1yi = 1 x j and from (3.13.10).

PROBLEMS

Let (a,) be an arbitrary sequence in a normed space E ; show that there exists a sequence 0, and a strictly increasing sequence (kJ of (x,) of points of E such that lim x,

+ + + "-0

integers such that (I= , xo x1 . . . x x nfor every n. Let u be a bijection of N onto itself, and for each n , let ~ ( nbe) the smallest number of intervals [a, 61 in N such that the union of these intervals is cs([O, n]) (a) Suppose p is boimcied in N. Let (x,) be a convergent series in a norined space E; show that the series (x0(J is convergent in E and that

m

"=a

m

x,

=

n=o

xSCn).

(b) Suppose p is unbounded in N. Define a series (x,) of real numbers which is convergent, but such that the series (rS(,,Jis not convergent in R . (Define by induction on k a strictly increasing sequence (mk)of integers having the following properties: (1) If n, is the largest element of u([O, i n x ] ) ,then [0, n k ] is contained in u([O, mk+,]). (2) p(md > k 1. Define then x,, for nk < n < n k + , such that x. = 0 except for 2k conveniently chosen values of n , at each of which x,, is alternately equal to Ilk or to - l / k . ) Let (x,) be a convergent series in a normed space E ; let u be a bijection of N onto itself, and let r(n) = lu(n) - nl . sup \\xn,\\.

+

man

3 ABSOLUTELY C O N V E R G E N T SERIES Show that if lirn r ( n ) = 0, the series (xO{"))is convergent in E and that n-

m

c x0("))- c xy for large n.)

m

97

c xO("). m

n=O

x, =

n=O

n

(Evaluate the difference

k=O

k=O

Let (x,J (m> 0, n > 0) be a double sequence of points of a normed space E. Suppose that: ( I ) for each m > 0, the series x , , ,-t ~ x , , , ~ . . . x,, . . . is convergent in E; .. . ; ( 2 ) for each n > 0, the series let y,, be its sum, and let Y,,,,= x,,,, + x ,,,," TO" YI" . . Y,,,, . . is convergent in E; let t . be its sum. x,, . is convergent; let (a) Show that for each n > 0, the series xOn xI, . z, be its sum.

+

+

+

+ +. + +.

(b) In order that

c

+

+ +. +

y,,, =

,n=0

(a) Show that theseries

+..

c z,, it is necessary and sufficient that lim t, 0. 1 is convergent and has a sum equal to c m2-n2

n=o

n-m

=

-

3/4m2

n31,n;fm

(decompose the rational fraction l/(m' - x')). 1 (b) Let u,"" = -if m # n, and N,,= 0; show that m' - nz

5(2

n = o U,")

n,=0

E( f

= - n=o

, = o U",") # 0.

I f f is a function defined in N x N, with values in a metric space, we denote by f ( m , n ) the limit o f f (when it exists) at the point (+to, to) of a x 8, with lirn

+

m-m."-m

respect to the subspace N x N (Section 3.13). Let (x,") be a double sequence of real numbers, and let s,, = C XIk. h 6 m , kQn

(a) If

lim s,,,,exists, then

m-m,n-a,

lirn

m-m,n-+m

xmn= 0. Give an example in which xnn,= x m n ,

~ , ~ , ~ ~ = - x , ~ , ~ ~ + ~ = - ~ , ~ + ~ , ~ ~ f o r m ~ 2 n +that 1 , x lirn ~ . , ~ sm,=O, .=O,such

+

+

+

+ +

+.

m-rm,n-tm

+

+.

and none of the series xmz0 xml . . . x,." . . ,xOn x l , f.. . x,,, . * is convergent. (b) Give an example in which xnvn= 0 except if m = n 1 , m = n or n = rn 1

c x,,,,, m

(hence all series m, n, but

lirn

",+ m , n-

n=O m

,,s

m

,n=o

x,,,, are convergent),

c x,, c m

m

=

"=O

,=O

+

=0

for all indices

does not exist.

3. A B S O L U T E L Y C O N V E R G E N T SERIES

(5.3.1) In order that a series (x,) of positive numbers be convergent it is necessary and sufJjcie17t that for a strictly increasing sequence (k,) of integers

1x, W

3 0, the sequerice (sk,,) of partial sums be majorized, and then the sum s = is equal to sup sk,.

n=O

n

The assumption x, 3 0 is equivalent to s , - ~ < s, follows at once from (4.2.1).

and then the result

98

NORMED SPACES

V

In a Banach space E, an absolutely convergent series (x,) is a series such that the series of general term llxnll is convergent.

(5.3.2) In a Banach space E , an absolutely convergent series (x,) is convergent,

By assumption, for any and anyp 2 0, IIxn+lII

+

E

> 0, there is an integer no such that for n 2 no

+ IIX,,+~II <

IIxn+1

E;

hence

+ ... + xn+pll< E ,

which proves the convergence of (x,) by (5.2.1). Moreover, for any n, llxo

+ xn)I< llxoll +

+

+ l[x,,Il;

the

inequality

I( F x.11 < c llxnll m

n=O

n=O

then follows from the principle of extension of inequalities (3.1 5.4).

(5.3.3) Zf(x,) is an absolutely convergent series and 0 n bijection of N onto itself, then (y,,), with y , = x0(,,,, is an absolutely convergent series, and

c x, c y , ("commutativity of absolutely convergent series). Let 1xk, s; c y,; for each n, let m be the largest integer in the set a@, n]); then by definition llykll < c IIxill, and (5.3.1) shows that m

m

=

n=O

"

n=O

n

n

sn =

=

k=O

k=O

n

rn

k=O

i=O

(y,) is absolutely convergent. Moreover, for any E > 0, let no be such that IIxn-lll + + I I X , , + ~ \ ~ < E for n 2 no and p 2 0; then if m, is the largest integer in a-'([O, no]), we have (Iyn+lll+ + Ilyn+pll< E for n 2 m,, p 2 0; furthermore the difference sko - s, is the sum of terms xi withj > n o , hence IIsko - sno\l< E ; therefore, for n 2 no and n 2 m,, 11s; - snII < 3 ~ , which proves that

c x, m

n=O

a,

= n=O

y, .

Let A be any denumerable set. We say that a family ( x , ) , ~ of ~ elements of a Banach space E is absolutely summable if, for a bijection cp of N onto A, the series (xlPcn,) is absolutely convergent; it follows from (5.3.3) that this property is independent of the particular bijection cp, and that we can define the sum of the family (x,),,~ as

c xlP(,,),which we also write c x u . As any m

n=O

asA

denumerable set S c E can be considered as a family (with S as the set of indices) we can also speak of an absolutely summable (denumerable) subset of E and of its sum.

3 ABSOLUTELY CONVERGENT SERIES

99

(5.3.4) In order that a denumerable family ( x , ) , , ~of elements of a Banach space E be absolutely summable, a necessary and sufJicient condition is that )Ix,JI (J c A andfinite) be bounded. Then, for any E > 0, the jinite sums

1

U E l

there exists a jinite subset H of A such that, for any jinite subset K of A for which H n K = 0, IIxull < E , and for any jinite subset L 3 H of A,

1

The first two assertions follow at once from the definition and from (5.3.1). Then, for any finite subset L 2 H, we can write L = H u K with H n K = 0, hence 1) x x U - E x a l l < E ; from the definition of the sum aeH

aeL

1xu it follows (after ordering A by an arbitrary

bijection of N onto A)

(5.3.5) Let ( x , ) , , ~be an absolutely summable family of elements of a Banach space E. Then,jor every subset B of A, the family(xa),eBisabsolutely summable, and C IIxaII G E IIxaII. UEA

U E B

If B is finite, the result immediately follows from the definition. If B is infinite, then llxall < 1 Ilx,Il for each finite subset J of B, and the

x

Or01

result follows from (5.3.4).

aeA

(5.3.6) Let (x,),,~be an absolutely summable family of elements of a Banach space E. Let (B,) be an injinite sequence of nonempty subsets of A, such that A= B,, and B, n B, = 0for p # q ; then, if z, = 1 x u , the series (z,)

u n

is absolutely convergent, and convergent series).

k

m

1z, = C x,

n=O

ueB,

(“associativity”

of absolutely

aeA

Given any E > 0 and any integer n, there exists, by (5.3.2), for each a finite subset J, of B, such that llZkI/ < llxull + E / ( H 1); if

J=

n

+

E

< n,

asJk

J k , we have therefore

1 Ilz,(l < 1llxull + n

k=O

k=O

aE l

E

G

1 IIxaII 4-

aeA

E;

(5.3.1)

then proves that the series (z,) is absolutely convergent. Moreover, let H be a finite subset of A such that, for any finite subset K of A such ( ( E , whence, for any finite subset L of A containthat H n K = 0, ( ( x a < ing H,

11 1xu aeA

c x,I aeK

aEL

< 2~ (see (5.3.4)). Let 170

be the largest integer such

100

V

NORMED SPACES

that H n B,, # @, and let n be an arbitrary integer 2 n o . For each k < n, let Jk be a finite subset of B, containing H n B,, and such that for any finite subset Lk of B, containing J,, we have llzk - C x,II < d ( n 1) (5.3.4). Then, if L =

u Lk,we have /I

from the definition of H that

I<<

zk - C xu

n

k=O

U E

C zk - C x,

1Ik:o"

E,

+

Lk

and as L 3 H, it follows

3 ~which , ends the proof.

u : r l l

There is a similar (and easier) result when A is decomposed in afinite number of subsets B, (1 < k < n ) ; moreover, in that case, there is a converse to (5.3.6), namely, if each of the families (xu)uEBk is absolutely summable, so is (xu)uEA;the proof follows, by induction on n, from the criterion (5.3.4).

PROBLEMS

1. Let (d,,)be a sequence of real numbers d, 2 0, such that the series (d,) is not convergent (i.e., lim

n

1ilk = +a).What can be said of the convergence of the following series:

k=o

n-m

d,. l-td"'

2.

d. l+nd.'

.

.

dl8

1

d.

+ nv,,'

i - Q -?

Let (u,) be a convergent series of real numbers, which is not absolutely convergent, and m

let s = C u.. For each numbers' 2 s,show that there exists a bijection u of N onto itself "=O

such that u(n) = n for all n such that u.

m

> 0, and that C us(")= s'.

(Show by induction

"=O

that for each n there is a bijection unof N onto itself such that u,(k) = k for all k such , is an index p n having the property that, for that uk 2 0 and that, if up)= u , , , ( ~ )there k3p,

I

3'-

cup'I < l / n ;

furthermore, is such that uncl(k)= an@) for all k such that u,(k) < p I and all k Such that U k < - I/n.) 3. Show that for every finite family of points of the product space R" (with the one has llxlII < 2n . sup llcxill (consider norm ljxll= sup [&I for x =

c

1EI

IcL

~ E J

first the case n = 1). 4. In a normed space E, a series (x.) is said to be comrnutatively convergent if, for every bijection u of N onto itself, the series (x,,(~))is convergent. (a) In order that a convergent series (x.) be comrnutatively convergent, it is necessary and sufficient that for every E > 0, there exist a finite subset J of N such that, for any subset H of N for which J n H = x.ll< E . When that condition is satisfied, the

a, IIc

sum

m n=O

"EH

xocn)is independent of u. (To prove the last assertion, and the sufficiency of the

3 ABSOLUTELY CONVERGENT SERIES

101

condition, proceed as in (5.3.3). To show that the condition is necessary, use contradiction: there would exist an a > 0 and an infinity of finite subsets H, (k = 1,2, ..) of N, no two of which have common points, and such that I/ x.l/> tc for each k ; starting

.

nEHk

from the existence of these subsets, define u for which the series (x,,,,) is not convergent.) (b) Suppose the series (x.) is such that, for any strictly increasing sequence (nk) of integers, the series (xnJ is convergent. Show that the series (x.) is commutatively convergent (use the same argument as in (a)). Prove the converse when E is complete (use the criterion proved in (a)). (c) If E = R", show that any commutatively convergent series in E is absolutely convergent (use Problem 3 and the criterion of (a)). (d) Extend the associativity property (5.3.6) to commutatively convergent series. 5. Let E be the real vector space consisting of all infinite sequences x = (&).,o of real numbers, such that lim 6. = 0. For any x E E, let jJx)I= sup I&,\. "-+

m

n

(a) Show that llxll is a norm on E, and that E, with that norm, is a Banach space (the " space (co) " of Banach). (b) Let embe the sequence (8,,,n)n20,with 8,. = 0 if m # n,8,,,", = 1. Show that forevery point x

=

(6.)

E E,

m

I t 5 . e nis commutatively convergent

the series

in E, and that its

"=O

sum is x ; give examples in which the series is not absolutely convergent. 6. (a) Let (s.) an increasing sequence of numbers > O which tends to co. Show that the series of general term (s. - S"-~)/S" has an infinite sum. For each number p > 0, show that the series of general term (s. - s.- l)/s,,s,p- is convergent; compare to the series of general term

+

1

1

sp.-1

sf: m

(b) Let (uJnao be a sequence of numbers 2 0 such that uo > 0, and let s. = c u k t=o

for each n 2 0. Show that a necessary and sufficient condition for the series of general term u. to be convergent is that the series of general term u./s. be convergent (use (a)). 7. Let (u.) be a convergent series of numbers 3 0 . Show that there exists an increasing sequence (c.) of numbers >0, such that lim c,, = +a,and that the series (c. u.) is II-

m

convergent. 8. Let (u,) be a convergent series of numbers 30.Show that lim

u1

"-.m

+ 2n* + .. . + nu. = O n

and

c

n=1

*

write 1 =--1 ) . n(n + I ) n N 1

+

u1 + 2 u 2 +...+nu.

n(n+ I )

=xu" n=l

102

V NORMED SPACES

4. SUBSPACES A N D FINITE PRODUCTS O F NORMED SPACES

Let E be a normed space, F a vector subspace of E (i.e. a subset such that x E F and y E F imply CLX By E F for any pair of scalars a, B); the restriction to F of the norm of E is clearly a norm on F, which defines on F the distance and topology induced by those of E. When talking of a “subspace” of E, we will in general mean a vector subspace with the induced norm. If E is a Banach space, any closed subspace F of E is a Banach space by (3.14.5); conversely, if a subspace F of a normed space E is a Banach space, F is closed in E by (3.14.4).

+

(5.4.1) I f F is a vector subspace of a normed space E, its closure vector subspace.

F in E is a

+

By assumption, the mapping ( x , y ) + x y of E x E into E maps F x F into F, hence maps F x F into F, by(3.11.4); as F x F = P x I‘ by (3.20.3), the relations x E F, y E F imply x y E F. Using the continuity of (A, x ) +Ax, we similarly show that x E F implies Ax E F for any scalar A.

+

We say that a subset A of a normed space E is total if the (finite) linear combinations of vectors of A form a dense subspace of E ; we say that a family (x,) is total if the set of its elements is total. Let El, E, be two normed spaces, and consider the product vector space E = El x E2 (with ( x l , x 2 ) (y,, y,) = ( x l yl, x 2 y,) and A(xl, x 2 ) = (Axl, Ax,)). It is immediately verified that the mapping (xl, x,) + sup (\lxl\l, llx211) is a norm on E, which defines on E the distance corresponding to the distances on El, E2, and therefore the topology of the product space El x E, as defined in Section 3.20. The “natural” injections x1 + (xl, 0), x 2 + (0, x,) are linear isometries of El and E, respectively onto the closed subspaces E; = El x { 0 } ,E; = (0) x E, of E(3.20.11), and E is the direct sum of its subspaces E;, E; , which are often identified to El, E, respectively. Conversely, suppose a normed space E is a direct sum of two vector subspaces F1, F, ;each x E E can be written in a unique way x = p l ( x ) p,(x), with p l ( x ) E F1, p,(x) E F2 , and p 1 , p 2 are linear mappings of E into F,, F, respectively (the “projections” of E onto F,, F,). The “ natural” mapping (yl, y,) + yl y 2 is a linear bijection of the product space F, x F, onto E, which is continuous (by (5.1.5)), but nor necessarily bicontinuous (see Section 6.5, Problem 2).

+

+

+

+

+

5 CONDITION

OF CONTINUITY OF A MULTILINEAR MAPPING

103

+

(5.4.2) In order that the mapping (y,, y,) + y1 y , be a homeomorphism of F, x F2 onto E, a necessary and srrficient condition is that one of the

linear mappings p,, p2 be continuous.

+

Observe that as x = pl(x) p2(x),if one of the mappings p l , p 2 is continuous, so is the other. The mapping x + (pl(x),p2(x))of E onto F, x F, being the inverse mapping to (yl, y,) + y , y 2 , the conclusion follows from (3.20.4).

+

When the condition of (5.4.2) is satisfied, E is called the topological direct sum of F,, F,; a subspace F of E such that there exists another subspace G for which E is the topological direct sum of F and G is called a topological direct summand of E, and any subspace G having the preceding property is called a topological supplement to F. Any topological direct but there may exist closed summand is necessarily closed (by (3.20.1I)), subspaces which are not topological direct summands (although any subspace always has an algebraic supplement in E); for examples of such spaces see Bourbaki [6],Chapter IV, p. 119, Exercise 5c, and p. 122, Exercise 17b. The definitions and results relative to the product of two normed spaces are immediately extended to the product of a finite number n of normed spaces (by induction on n).

5. C O N D I T I O N OF C O N T I N U I T Y OF A M U L T I L I N E A R MAPPING

(5.5.1) Let El, . . . , En be n normed spaces, F a normed space, u a multilinear mapping of El x * * x En into F. In order that u be continuous, a necessary and suficient condition is the existence of a number a > 0 such that, for any (x1,..., x n ) € E 1x E , x - * * x E n ,

-

IlU(X1, x 2 ,

.

* * 9

x,)ll d a

- II x1 II

*

11x2

II *

-

*

IIxiiII.

We write the proof for n = 2. 1. Suficiency. To prove u is continuous at any point (c,, c,), hence we write u(xl,x,) - u(c,, c,) = u(xl - cl, x 2 ) u(cl, x , - c,), IIu(xl, x 2 ) - u(cl, c2)ll d a(llx, - clII * llxzII + llclII llx2- czll). For any 6 such that 0 < 6 < 1, suppose llxl - c1 11 d 6 , IJxz- c211 d 6, hence ((x2< ( ( \lc,ll 1. We therefore have

-

+

+

IIu(x1, x2) - 4c1,

which is arbitrarily small with 6.

c2)ll

,< a(llc1II

+ IIc211 + 1)6,

V

104

NORMED SPACES

2. Necessity. If u is continuous at the point (0, 0), there exists a ball B: sup (l\xll\, Ilx2\l)< r in El x E2 such that the relation (xl, x2)E B implies [Iu(xl,x2)ll < 1. Let now (xl, x 2 ) be arbitrary; suppose first xI# 0, x2 # 0; then if z1 = r x l / ~ ~ xzl2~= / , rx2/I/x2JJ, we have IIzlll = IJz21(= r , and therefore Ilu(z,, z2)II < 1. But u(zl, z 2 ) = r2u(x1,x2)/llxlll * IIx211, and therefore IIu(xl, x2)I(< a * (Ixl(I* llx211 with a = l/r2. If x1 = 0 or x2 = 0, u(xl, x2) = 0, hence the preceding inequality still holds.

(5.5.2) Let u be a continuous linear mapping of a Banach space E into a Banach space F. If (x,) is a convergent (resp. absolutely convergent) series in E, (u(x,)> is a convergent (resp. absolutely convergent) series in F, and C u(xn) = ~ 0 ) n

n

The convergence of the series (u(x,)) and the relation

u(x,) n

=u(c

x,)

n

follow at once from the definition of a continuous linear mapping (see (3.13.4)). From (5.5.1) it follows that there is a constant a > 0 such that IIu(x,)I( < a . llxnllfor every n, hence the series (~(x,))is absolutely convergent by (5.3.1) if the series (x,) is absolutely convergent.

(5.5.3) Let E, F, G be three Banach spaces, u a continuous bilinear mapping of E x F into G. If(x,) is an absolutely convergent series in E, (y,) an absolutely convergent series in F, then the family (u(xm,y,)) is absolutely summable and

Using the criterion (5.3.4), we have to prove that for any p , the sums (Iu(xm, y,,)II are bounded. But from (5.5.1), there is an a > 0 such that

1

mQp,n$p

lIU(Xm

9

yn>lI < allxmll * IlYnIl, hence

which is bounded, due to the assumptions on (x,) and (y,,). Moreover from

(5.3.6) and (5.5.2) it follows that, if s = 1x,, s' = 1 yn , n

n

5 CONDITION OF CONTINUITY OF A MULTILINEAR MAPPING

105

(5.5.4) Let E be a normed space, F a Banach space, G a dense subspace of E , a continuous linear mapping of G into F . Then there is a unique continuous linear mappingf of E into F which is an extension off. From (5.5.1) it follows that f is uniformly continuous in G , since = 11 f ( x - y)II < a . IIx - yll; hence by (3.15.6) there is a unique continuous extension f o f f t o E. The fact that f is linear follows from (5.1.5) and the principle of extension of identities (3.1 5.2).

IIf ( x ) -f(y)II

PROBLEMS

+

1. Let u be a mapping of a normed space E into a normed space F such that u(x y ) = u ( x ) u(y) for any pair of points x , y of E and that u is bounded in the ball B(0; 1) in E; show that u is linear and continuous. (Observe that u(rx) = ru(x) for rational r , and

+

that for y

E B(0;

ll( + 3 I : -y

l), u x

- u(x)

Q -l]u(y)ll

for every integer n 3 1 ; conclude

that u(hx) = hu(x) for every real h, by taking y = n(r - h)x, where r is rational.) 2. Let E, F be two normed spaces, u a linear mapping of E into F. Show that if for every sequence (x.) in E such that lim x, = 0, the sequence ( ~ ( x , , )is) bounded in F, then u n-rm

is continuous. (Give an indirect proof.) 3. (a) Let a , b be two points of a normed space E. Let B1 be the set of all x E E such that IIx - all = Ilx - bll= ((a- 6\1/2; for n > 1, let B. be the set of x E Bn-l such that IIx - yll < 6(B,-,)/2 for all y E Bn-l(&A) being the diameter of a set A). Show that 6(B,) < 6(B.-,)/2, and that the intersection of all the B. is reduced to (a b)/2. (b) Deduce from (a) that iffis an isometry of a real normed space E onto a real normed space F, thenf(x) = u(x) c, where u is a linear isometry, and c E F. 4. Let us call rectangle in N x N a product of two intervals of N; for any finite subset H of N x N, let $(H) be the smallest number of rectangles whose union is H. Let (H.) be an increasing sequence of finite subsets of N x N, whose union is N x N and such that the sequence ($(H.)) is bounded. Let E, F, G be three normed spaces, (x.) (resp. (y,)) a convergent series in E (resp. F),fa continuous bilinear mapping of E x F into G. Show that

+

+

(*)

5.

lim

n-m

C


("YO "I* ).

f ( x h , ~ J = f C x.,Cy.

Let (H.) be an increasing sequence of finite subsets of N x N, whose union is N x N; for eachj E Nand each n E N, let F ( j , n) bethesmallestnumberof intervalsof Nwhoseunion is theset H;'(j)ofall integersisuch that ( i , j ) e H n Suppose~0',n)isboundedinN . x N. Let (x.) be a convergent series in a normed space E, (y.) an absolutely convergent series in a normed space F, u a continuous bilinear mapping of E x F into a normed space G. Show that formula (*) of Problem 4 still holds (use (5.5.1), and remark that the sums xf are bounded in E for all j , n) (cf. Section 12.16, Problem 12). (f.J)EHn

6. Let E, F be two real normed spaces. A mapping f o f E into F is said to be linear in a neighborhood of0 if there exists a 6 > 0 such that: (1) the relations JIxJJ G 6, IIx'II Q 6 , IIx x'll Q 6 in E implyf(x x') = f ( x ) +f(x'); (2) the relations llxll 6 6, llhxll G 6 in E (with h E R) imply f(hx) = hf(x).

+

+

106

V

NORMED SPACES

(a) Show that iffsatisfies condition (1) and is continuous at the point 0, it is continuous in a neighborhood of 0 and is linear in a neighborhood of 0 (cf. Problem 1). (b) Let g be a mapping of E into F; in order that g be continuous at the point 0 and linear in a neighborhood of 0, a necessary and sufficient condition is that for any convergent series (x.) in E, the partial sums of the series (g(x.) be bounded in F. (To prove sufficiency, first observe that one must have g(0) = 0; if, for every n, there exist three elements u., v., w. of E such that llunll< 2-", llvnll < 2-", llwnll < 2-", u,, v. w. = 0 and g(uJ g(u,) +g(w.) # 0, form a series (x,) violating the assumption. If there are no such sequences u,, v., w., g verifies condition (1); show that it is necessarily continuous at 0.)

+ +

+

6. EQUIVALENT NORMS

Let E be a vector space (over the real or the complex field), llxlll and llxll, two norms on E; we say that llxlll isfiner than llxll, if the topology defined by (IxI(lis finer than the topology defined by IJxJI,(see Section 3.12); if we note El (resp. E,) the normed space determined by llxlll (resp. Ilxl12), this means that the identity mapping x + x of El into E, is continuous, hence, by (5.5.1), that condition is equivalent to the existence of a number a > 0 such that IIxIJ2< a - IJxJI1. We say that the two norms IIxII1, I(xI(, are equivalent if they define the same topology on E. The preceding remark yields at once: (5.6.1) In order that the two norms IIxlll, ((x((, on a vector space E be equivalent a necessary and sufficient condition is that there exist two constants a > 0,

b >O, such that allxll1

for any x E E.

< llxllz d bllxll1

The corresponding distances are then uniformly equivalent (Section 3.14). For instance, on the product El x E, of two normed spaces, the norms sup(IIxlII, IIx~II)~IIxlII + IIx211, (IIxlI12 + I I X ~ I I ~ ) are " ~ equivalent. On the space E = %?&), the norm l l f l l l defined in (5.1.4) is not equivalent to the norm Ilfll, = suplf(t)l (see Section 5.1, Problem 1). re1

7. SPACES O F C O N T I N U O U S MULTILINEAR MAPPINGS

Let E, F, be two normed spaces; the set Y(E; F) of all continuous linear mappings of E into F is a vector space, as follows from (5.1 S),(3.20.4), and (3.11.5).

7 SPACES OF CONTINUOUS MULTILINEAR MAPPINGS

107

For each u E Y(E; F), let IJuJI be the g.1.b. of all constants a > 0 which satisfy the relation IIu(x)II < a * llxll (see (5.5.1)) for all X. We can also write

(5.7.1) For by definition, for each a > IIuII, and llxll < I , IIu(x)I( 0, llxll 6 1

for any b such that 0 < b < IIuII, there is an x E E such that IIu(x)I( > bllxll; this implies x # 0, hence if z = x/llx/l, we still have IIu(z)II > b llzll = b, and as llzll = 1, this proves that b < sup IIu(x)I(,hence I(u(I< sup IIu(x)(I, 9

llxll 6 1

llxll 1

and (5.7.1)is proved. The same argument also shows that if E # (0)

(5.7.2) We now show that /lull is a norm on the vector space Y ( E ; F). For if u = 0, then IJuJJ= 0 by (5.7.1),and conversely if llull = 0, then u(x) = 0 for llxll ,< 1, hence, for any x # 0 in E, u(x) = llxllu(x/llxll)= 0. It also follows from (5.7.1) that IIAull = 111 * IJu/I; finally, if w = u u, we have II4dI IIu(x)II llu(x>II,hence llwll llull llull from (5.7.1).

<

(5.7.3)

+

<

+

+

If' F is complete, so is the normed space 8 (E; F).

For let (u,,) be a Cauchy sequence in 8 ( E ; F); for any E > 0, there is therefore an no such that [/urn- u,II < E for m 2 n o , n 2 n o . By (5.7.1), for any x such that IlxlI < 1, we therefore have Ilurn(x) - u,,(x)II < E for m 2 n o , n 2 no; this shows that the sequence (u,(x)) is a Cauchy sequence in F, hence converges to an element u(x) E F. This is also true for any x E E, since we can write x = Az with ( ( z (i ( I, hence u,(x) = Au,,(z) tends to a limit v(x) = Au(z). From the relation u,,(x+ y ) = u,,(x)+ u,,(y) and from (5.1.5) it follows that u(x + y) = v(x) + v(y), and one shows similarly that u(Ax) = Au(x), in other words u in linear. Finally, from Ilu,(x) - u,,(x)II < E for m 3 n o , n >, n o , we deduce IIu(x) - u,,(x)))< E for JJxJI ,< 1, hence IIu(x)II < Ilu,,ll + E , which proves (by (5.5.1)) that u is continuous, hence in 9 ( E ; F); furthermore JIu - u,)) < E for n 2 no (by (5.7.1)), which proves the sequence (u,,) converges to u. From the definition it follows that, for every x E E and every u E 9(E; F),

(5.7.4)

IIu(4Il 5 IIuII . IIXII

which proves that the bilinear mapping (x, u) --* u(x) of E x Y(E; F) into F is continuous (by (5.5.1)).

108

V

NORMED SPACES

The definition of the norm in 9 ( E ; F) depends on the norms in E and in F; but it is readily seen that, when the norms in E and F are replaced by equivalent norms (Section 5.6) the new norm in 9 ( E ; F) is equivalent to the old one. (5.7.5) Let u be a continuous linear niapping of a nornied space E into a normed space F, and u a coritinuous linear mapping of F into a nornwd space G. Then Ilu 0 ull 6 llull . llull.

For if Ibll < 1, then by (5.7.4) Il44x))Il Q l l ~ l .l IIu(x)/I G IIuIl * Ilull, and the result follows from (5.7.1). (5.7.6) I f F is a real (resp. coniplex) nornied space, the mapping ~ A i c hto each a E F associates the elernelit 6,: 5 +(a o f Y ( R ; F) (resp. 9 ( C ; F)) is a linear isometry of F onto 2 ( R ; F) (resp. 9 ( C ; F)).

The mapping a + 0, is obviously linear; it is surjective, for every linear mapping f of R (resp. C) into F is such t h a t f ( 0 = f ( < . 1) = (J’(1) = [ a with a = f ( l ) . Finally ~ ~ =O sup a ~ l\(ali ~ = I/ a I/ by axiom (111) of Section 5.1. l
1

Let now El, . . . , En,F ben f 1 normed spaces, and define 2’(E,, . . . , En; F) as the vector space of all continuous multilinear mappings of El x . . . x En into F. Then for u E 9 ( E l , . . . , En; F), the same argument as above shows that the g.1.b. \lull of all constants a > 0 such that IIu(x1, . . . * xn)ll 6 a l l x ~/I * 9

is also given by (5.7.7)

I1 1’ 1I =

SUP llXlIl
..., l l x , , l \ < l

114x1,

. . llxnll *

*.

9

X,>II.

We also see that ljulj is a norm on 9(El, . . . , En; F); but in fact these vector spaces can be reduced to spaces 9(X; Y): (5.7.8) For each u E 9(E, F; G ) and each x E E, let u, be the linear niappirig y + u(x, y ) . Then ii: x -+ ux is a linear continuous mapping of E into 9(F; G ) , and the mapping u + ii is a linear isonietrj, of 2(E, F; G ) onto 9 ( E ; 2’ (F; G ) ) .

We have llux(y>ll= llu(x,y>ll < llull . llxll . Ilyll, hence ux is continuous by (5.5.1); moreover lIuxll = sup IIu(x, y)ll, hence (2.3.7) IIY II

1

7 SPACES OF CONTINUOUS MULTILINEAR MAPPINGS

109

which proves that x + u , (which is obviously linear) is continuous, and u + ii is an isometry of 2’( E, F; G) into 9(E; 9(F; G)). Finally u -+ ii is surjective, for if u EY(E; Y ( F ; G)), then u : (x,y ) + (u(x))(y)is obviously bilinear, and as II(u(x))(y)ll Q lI4x)Il . l l ~ l Q l llull . Ilxll . llyll by (5.7.4), u is continuous, and u(x) = u,, which ends the proof. By induction on n, it follows that 9 ( E , , E,, . . . , En;F) can be naturally identified (with conservation of the norm) to 9(El ; 9(E2; . . . , 2’(En; F)) . . .). PROBLEMS

Let E be the space (cO) of Banach, defined in Section 5.3, Problem 5 ; we keep the notations of that problem. Let I I be a continuous linear mapping of E into R ; if u(e.) = q n , show that the series cqnis absolutely convergent, and that, in the Banach space E’=

1.

P ( E ; R),

c lq,, (apply m

lli111 =

n

(5.5.1) for suitable values of x

E

E). Conversely, for any

“=O

absolutely convergent series (qn)of real numbers, there is one and only one continuous linear mapping m

then ~ ( x =)

I/

of E into R such that //(en) = 7. for every n ; and if x =

m

1& e n E E, ”=O

q,&, (the space E’, with the norm defined above, is the “space 1’”of

ll=O

Banach). (b) As a vector space (without a norm) E’ can be considered as a subspace of E; show that the norm on E’ is strictly finer (Section 5.6) than the restriction to E’ of the norm of E. (c) Show that the space E” = Y(E’; R) of the continuous linear mappings of E’ into R can be identified with the space of all bounded sequences x = (Cn) of real numbers, with norm llxlj sup 15.1 (“space I “ ” of Banach; use the same method as in (a)). E can be :

considered as a closed subspace of E . (d) In the space E’, let P be the subset of all absolutely convergent series N = (7.) with terms 1)“ 0; any element of E‘ can be written u - u, where both u and u are in P; yet show that the interior of the set P is empty. 2. (a) Let E be the space (cO) of Banach, and let U be a continuous linear mapping of E m

into itself. With the notations of Problem 1 , let U (en)= 1 amnem ; show that: (1)

-.

lim ,I,

m

= 0;

(2) the series

c I L Z , , ~ ~ ~ is convergent for every

fl=O

m=O

lamn[ is finite.

m ; (3) sup m

n=O

(Same method as in Problem I(a).) Prove the converse, and show that the Banach space Y ( E ; E) can be identified with the space of double sequences U = (an,”) satisfying the m

preceding conditions, with the norm llUll= sup “a

n=0

lam,/.

(b) Let E’ be the space I’ of Banach (Problem 1). Show similarly that the Banach such space Y’(E’; E ) can be identified with the space of double sequences U = (amn) that: ( I ) the series

m

C Icc,,,~~is convergent

m =0

c1 m

norm is then equal to IIUll

SUP

”

m=O

for every n ; (2) sup

~ ~ ~ ~ 1 .

n

m m=O

lmmnl is finite; the

110

NORMED SPACES

V

3. Let E # {O] be a normed space; show that there cannot exist two continuous linear mappings u , v of E into itself such that u o u - u o u = I E . (Prove that this would imply II 0 unfl - u"+' 0 I I = ( n + t)v", and therefore the inequality ( n l ) l / u " ~<~ 2lIull. I l u I / . I/u"I/, which leads to v" = 0 as soon as n is large enough, hence u = 0, which is a contradiction.)

+

8. CLOSED HYPERPLANES A N D C O N T I N U O U S LINEAR FORMS

We recall that a linear f o r m on a real (resp. complex) vector space E is a linear mapping f of E into R (resp. C);its kernel H = f - ' ( O ) is then a vector subspace such that for any a $ H, E is the algebraic direct sum of H and Ra (resp. Ca). A subspace having this last property is called a hyperplane; if H is a hyperplane, a 6 H, and if for any x E E we write x = f ( x ) a y with f ( x ) a scalar and y E H, then f is a linear form and H =f -'(O). The relationf(x) = 0 is called an equation of H ; iffl is another linear form such that H = f i - ' ( O ) , then fi = af (a scalar). We also recall that a hyperplane is maximal: any vector subspace of E containing a hyperplane H is either H or E itself.

+

(5.8.1) In a real (resp. coniplex) normed space E, let H be a hyperplane of equationf(x) = 0. In order that H be closed in E, a necessary and suflcient condition is that ,f be continuous. For any b E H , E is then the topological direct sum (see Section 5.4) of H and of the one-diniensional subspace D = Rb (resp. D = Cb).

It is clear that i f f is continuous, H = f - ' ( O ) is closed (see (3.15.1)). To prove the converse, let a 4 H be such that f ( a ) = 1. As H is closed, so is a H (by (5.1.5)), and as 0 4 a + H, there is a ball V: l\xl\< r which does not meet a + H ; therefore x E V implies f ( x ) # 1 . We prove x E V implies If(x)l d 1. Suppose the contrary, and let c1 =f(x), with Ic(( > 1 ; then Ilx/aIl = (l/lctl)llx\l < r , andf(x/a) = 1, which contradicts the definition of V. By homogeneity and (5.5.1) it follows that f is continuous. If b $ H, we have x = g(x)b + y with y E H for each x E E, and g ( x ) = 0 is another equation of H ; hence g is continuous, and the mapping x + g ( x ) b of E into D = R b (resp. Cb) is therefore continuous, which proves the last part of (5.8.1) by (5.4.2).

+

(5.8.2)

For

In a normed space E, a hyperplane H is either closed or dense.

is a vector subspace (by (5.4.1)) which can only be E or H.

9 FINITE DIMENSIONAL NORMED SPACES

111

PROBLEMS

1. Let E be the (noncomplete) subspace of the space (co) of Banach, consisting of the sequences x = (6.) of real numbers, having only afinire number of terms different from 0. For any sequence (an)of real numbers, the mapping x -+ u(x) =

c m

En

GC. is

a linear

"=O

form on E, and all linear forms on E are obtained in that way; which of them are continuous (see (5.5.4) and Problem 1 of Section 5.7)? 2. (a) In a real normed space E, let H be the closed hyperplane of equation u(x) = 0, where u is a continuous linear form. Show that for any point a E E, the distance 4 a , n = 144 I illrrll. (b) In the space (co) of Banach, let H be the closed hyperplane of equation u(x) =

c 2-"& m

= 0;

if a # H, show that there is no point b E H such that d(a, H) = d(a, b).

"=O

3. In a real vector space E, the linear varieties of codimension 1 (Section 5.1, Problem 5) are again called hyperplanes; they are the sets defined by anequation ofthe type u(x) = a, where u is a linear form, a any real number; the hyperplanes considered in the text are those which contain 0, and are also called homogeneous hyperplanes; any hyperplane defined by an equation u ( x ) = a is said to be parallel to the homogeneous hyperplane defined by u(x) = 0. If A is a nonempty subset of E, a hyperplane of support of A is a hyperplane H defined by an equation u(x) = a , such that u(x) - a > 0 for all x E A, or u(x) - u < 0 for all x E A, and u ( x o ) = a for at least one point xo E A. (a) In a real normed space E, a hyperplane of support of a set containing a n interior point is closed (see (5.8.2)). (b) Let A be a compact subset of a real normed space E; show that for any homogeneous closed hyperplane H, defined by the equation u ( x ) = 0, there are two hyperplanes of support of A which are defined by equations of the form u(x) = a, and may eventually coincide; their distance is at most equal to the diameter of A. (c) In the space (c,) of Banach, consider the continuous linear form x + u ( x ) = m

2-"5, ; show that the closed ball B'(0; 1) has no hyperplane of support having an

"=O

equation of the form u ( x ) = a (cf. Problem 2(b)).

9. F I N I T E D I M E N S I O N A L N O R M E D SPACES

(5.9.1) Let E be an n-dimensional real (resp. complex) normed vector space; q(a,, . . . , a,) is a basis of E, the mapping

(51, of

* *

1

> Cn)

4

+

5 1 ~ 1

* * *

+ ("an

R" (resp. Cn) onto E is bicontinuous.

We use induction on n, and prove first the result for n = 1. We know by (5.1.5) that 5 + [al is continuous; as a, # 0 and IItalll = llalI1 * 151, we have 151 < (l/llalII) * \15al\\,which proves the continuity of 5al + 5, by (5.5.1).

112

V

NORMED SPACES

Suppose the theorem is proved for n - I , and let H be the hyperplane in E generated by a,, . . . , a,,-l; the inductive assumption implies that the norm on H (induced by that of E) is equivalent to the norm sup Iti]; I Ci
hence H is coniplete (for both norms) and therefore closed in E (by (3.14.4)). It follows from (5.8.1)that the mapping ( t l a , + + <,a,) -+ is continuous, and this, together with the inductive assumption, ends the proof (by (3.20.4)and (5.4.2)). e . 1

<,

(5.9.2) I11 a normed space E, let V be a closed subspace, W aJinite diinensional subspace; then V W is closed in E. 0 1 particular, any Jinite dimensional subspace is closed in E.

+

We can use induction on the dimension n of W, and therefore reduce the proof to the case n = 1. Let W = Ra (resp. W = C a ) ; if a E V , V + W = V and there is nothing to prove. If not, we can write any x E V + W in the form x = , f ( x ) a+ y with y E V , and as V is a closed hyperplane in V + W, ,f is continuous in V + W, by (5.8.1). Let (x,) be a sequence of points of V W tending to a cluster point b of V + W (see (3.13.13)); write x, =f(x,)a y , . By (5.5.1),the sequence (,f(x,)) is a Cauchy sequence in R (resp. C), hence tends to a limit 2; therefore p, = x, -,f(x,)a tends to b - l a ; but as V is closed, the limit of (y,) is in V , hence b EV + W. Q.E.D. (See Section 6.5, Problem 2.)

+

+

(5.9.3) In a nornied space E, let V be a closed subspace ofJiiiite codirriension (i.e. having a Jinite dinzeiisional algebraic suppler?ient); then uny algebraic supplement of V is also a topological supplement.

Let W be an algebraic supplement of V in E ; we use induction on the dimension 11 of W, the result having been proved for n = I in (5.8.1). We can write W = D + U where D is one-dimensional and U is (11 - 1)-dimensional (direct sum); by (5.9.2), V + D is closed in E, hence U is a topological supplement to V + D by the inductive assumption. In other words, E is naturally homeomorphic to (V + D) x U ; by (5.8-1), V + D is naturally homeomorphic to V x D, hence E is naturally homeomorphic to V x D x U. Finally, as D x U is naturally homeomorphic to W, E is naturally homeomorphic to V x W. Q.E.D. (F. Riesz’s theorem) A locally compact nortned space E i s j n i t e dimensional. (5.9.4)

9 FINITE DIMENSIONAL NORMED SPACES

113

Replacing the norm by an equivalent one, we may suppose the ball B:

Ilxll < 1 is compact. Therefore (3.16.1) there exists a finite sequence of points a , (1 < i < f 7 ) such that B is contained in the union of the balls of center a, and radius 1/2. Let V be the finite dimensional subspace generated by the ui. We prove by contradiction that V = E. Suppose indeed there is an x E E which is not in V. As V is closed (by (5.9.2)) d(x,V) = a > 0; by definition of d(x, V), there is in V a pointy such that a < IIx - yll < +a. Let z = (x - y)/Ilx - yll; we have lizll = 1, hence there is an index i such that Ijz - aijl

< 1/2. Let us write x = Y + Ilx - ~ 1 = Y1 +~ Ilx - yllai + Ilx - ~ l l ( z- ail

+

and note that y (Ix- ylla, E V. By definition of d(x,V), we have therefore IIx - yll . Ilz - ail/ 2 a, hence I/x - y/l 2 2a, which contradicts the choice of y , since a # 0. PROBLEMS

1. Show that if E is a finite dimensional normed space, every linear mapping of E into a normed space F is continuous (use (5.9.1) and (5.1.5)). such that any element 2. We recall that a vector basis of a vector space E is a family of E can be written in a unique way as a linear combination of a finite number of aA; this implies in particular that the a, are linearly independent. (a) Let (an)be a sequence of linearly independent elements in a Banach space E. Define inductively a sequence (p,)of real numbers > O in the following way: if d, is the distance of the point p.a. to the subspace V.-, generated by a , , . .. ,a,..l (note that d. > 0

by (5.9.2)), take p.+, such that Ipn+,I* ~ ~ u n<+dJ3. , ~ ~Show that the series

rn

“=I

p.a.

.

is absolutely convergent, and that its sum x does not belong to any of the subspaces V, (b) Deduce from (a) that a Banach space of infinite dimension cannot have a denumerable vector basis. 3. Show that a normed space in which there is a sphere which is compact is finite dimensional. (Observe that the set of points in a normed space E such that a < IIxII S b (with a > 0) is homeomorphic to the product space of the interval [a, b] and of the sphere S : Ilxll = 1; use then Riesz’s theorem (5.9.4).) 4. Let E be a real normed space of finite dimension n , and let A be a compact subset of E. With the notations of Section 3.16, Problem 4, show that there exists a constant a > 0 such that N,(A) < a . ( l / ~ ) ” if; in addition A has an interior point, show that there exists a constant h > O such that M,,(A)> b . ( I / & ) ” ; in that case, one has H,(A) , . C,(A) n log I / & . 5. Let E be a real normed space of infinite dimension, and let (En) be a strictly increasing sequence of vector subspaces of E of finite dimension. Show that there exists a sequence (x.) of points of E such that x , E En, ~ ~ =xI and n ~d(x,, ~ En-,) = 1. Let be a function > O defined for f 2 1 , strictly decreasing and such that lim ( @ ) = O , and let z,=

-

t-+m

2 9 ( n ) x , . If K is the compact subset of E consisting of 0 and the z , , show that M,(K) 2 # ( E ) , where # is the inverse function of rp (defined in a neighborhood of 0); M,(K) can thus increase arbitrarily fast when I/& tend to +a.

114

V

NORMED SPACES

10. SEPARABLE NORMED SPACES

(5.1 0.1) I f i n a normed space E there exists a total sequence (Section 5.4),E is separable. Conversely, in a separable normed space E, there exists a total sequence consisting of linearly independent vectors.

Suppose (a,,)is a total sequence, and let D be the set of all (finite) linear combinations rlal . rnanwith rational coefficients (when E is a complex vector space, by a "rational" scalar we mean a complex number c1 i j , with both c1 and fi rational). D is a denumerable set by (1.9.3) and (1.9.4). As by definition the set L of all linear combinations of the a,, is dense in E, all we have to prove is that D is dense in L, and as

+

+

+

this follows from (2.2.16). Suppose conversely E is separable; we can of course suppose E is infinite dimensional (otherwise any basis of E is already aJinite total subset). Let (a,,)be an infinite dense sequence of vectors of E. We define by induction a subsequence (a,")having the property that it consists of linearly independent vectors and that for any m < k, , a,,, is a linear combination of a k l ,. . . , ak,. To do this, we merely take for k , the first index for which a,, # 0, and for k,,+,the smallest index m > k, such that a,,, is not in the subspace V,, generated by a k l ,. .., a,"; such an index exists, otherwise, as V,, is closed by (5.9.2), V,, would contain the closure E of the set of all the a , , contrary to assumption. It is then clear that (a,") has the required properties, and is obviously by construction a total sequence.

PROBLEM

Show that the spaces (co) and I' of Banach (Section 5.3, Problem 5, and Section 5.7, Problem 1) are separable, but that the space I" (Section 5.7, Problem 1) is not separable. (Show that in I" there exists a nondenumerable family ( x J of points such that IlxA- x ujl = 1 for h # p , using Problem 2(b) of Section 4.2, and (2.2.17).)

CHAPTER VI

HILBERT SPACES

Hilbert spaces constitute at present the most important examples of Banach spaces, not only because they are the most natural and closest generalization, in the realm of “ infinite dimensions,” of our classical euclidean geometry, but chiefly for the fact that they have been, up to now, the most useful spaces in the applications to functional analysis. With the exception of (6.3.1),all the results easily follow from the definitions and from the fundamental Cauchy-Schwarz inequality (6.2.4).

1. H E R M I T I A N FORMS

For any real or complex number A, we write X for its complex conjugate (equal to A if A is real). A hermitiun form on a real (resp. complex) vector space E is a mapping f of E x E into R (resp. C) which has the following properties : (1)

f(x + x‘,Y ) =f(x, Y ) + f ( x ’ , Y ) ,

(W

+ Y’) = A x ,Y ) +m, Y’), f ( kr) = Af(x,Y ) ,

(W

f ( x , AY) = m x ,Y>,

(11)

(V)

f(x, Y

f ( Y >4

=f(% Y ) .

(Observe that (11) and (IV) follow from the other identities; (V) implies that f(x, x) is real.) When E is a real vector space, conditions (I) to (IV) express that f is bilinear and (V) boils down to f ( y , x) = f ( x , y), which 115

116

VI

HILBERT SPACES

expresses t h a t f i s symmetric. For any finite systems ( x i ) ,( y j ) , (ai), (Bj), we have (6.1 .I)

by induction on the number of elements of these systems. From (6.1.1) it follows that if E is finite dimensional and ( a i ) is a basis of E, f is entirely determined by its values a i j = f ( a i , a j ) , which are such that (by V)) aJ,l. = a I,J.’

(6.1.2)

Indeed we have then, for x = (6.1.3)

1 S i a i ,y = i

S ( X .,Y>

viai I

C aij ti

i, i

iij

.

Conversely, for any system ( a i j ) of real (resp. complex) numbers satisfying (6.1.2), the right hand side of (6.1.3) defines on the real (resp. complex) finite dimensional vector space E a hermitian form. Example (6.1.4) Let D be a relatively compact open set in R2, and let E be the real (resp. complex) vector space of all real-valued (resp. complex-valued) bounded continuous functions in D, which have bounded continuous first derivatives in D. Then the mapping

(where a, h, c are continuous, bounded and real-valued in D) is a hermitian form on E. A pair of vectors x, y of a vector space E is orthogonal with respect to a hermitian form f on E if f ( x , y ) = 0 (it follows from (V) that the relation is symmetric in x, y ) ; a vector x which is orthogonal to itself (i.e.f(x, x) = 0) is isotropic with respect to$ For any subset M of E, the set of vectors y which are orthogonal to all vectors x E M is a vector subspace of E, which is said to be orthogorial to M (with respect t o f ) . It may happen that there exists a vector a # 0 which is orthogonal to the whole space E, in which case we say the formfis degenerate. On a finite dimensional space E, nondegenerate hermitian forms f defined by (6.1.3) are those for which the matrix ( a i j ) is invertible.

2

POSITIVE HERMITIAN FORMS

117

PROBLEMS

1. (a) Let f be a hermitian form on a vector space E. Show that if E is a real vector space, then 4f(x, Y> = f ( x

+ Y , x +v) -f(x

-Y, x

-Y)

and if E is a complex vector space

4fh, Y ) =f(x

+ y. x + y ) - f ( x

-Y ,x

-y)

+ i f ( x + iy, x + iy) - if(x - iy, x

- iy).

(b) Deduce from (a) that if f ( x , x ) = 0 for every vector in a subspace M of E, then f ( x , y ) = 0 for any pair of vectors x , y of M. (c) Give a proof of (b) without using the identities proved in (a). (Write that f(x hy, x +Xu) = O for any h.) 2. Let E be a complex vector space. Show that iffis a mapping of E x E into C satisfying conditions (I), (II), (III), (IV), and such that f ( x , x ) E R for every x E E, then f is a hermitian form on E.

+

2. POSITIVE H E R M I T I A N FORMS

We say a hermitian form f on a vector space E is positive iff(x, x) 2 0 for any x E E. For instance, the form q defined in example (6.1.4) is positive if a, b, c are 2 0 in D.

(6.2.1)

(Cauchy-Schwarz inequality)

I f f is a positive hermitian form,

then for any pair of vectors

X,

If(x, Y>12 < f ( x , X)f(Y, Y ) y in E.

Write a =f ( x , x), b = f ( x , y ) , c = f ( y , y ) and recall a and c are real and 2 0. Suppose first c # 0 and write that f ( x I y , x + l y ) 2 0 for any scalar 1, which gives a + b2 + bA + c3J 0 ; substituting I = -b/c yields the inequality. A similar argument applies when c = 0, a # 0 ; finally if a = c = 0, the substitution I = - h yields -266 2 0, i.e. b = 0.

+

(6.2.2) In order that a positii!e hermitian form f on E be nondegenerate a necessary and suficient condition is tliat there exist no isotropic vector for f otlier than 0 , i.e. that f ( x , x) > 0 for any x # 0 in E.

Indeed, f ( x , x) all y E E.

=0

implies, by Cauchy-Schwarz, that f (x, y ) = 0 for

118

(6.2.3)

VI

HILBERT SPACES

(Minkowski's inequality) I f f is a positi1.e hermitian form, tlien (f(x

+y, x +

d ( A x , x))''2 + ( f b ,

for any pair of vectors x, y in E.

+

As f ( x y , x equivalent to

+ y ) = f ( x , x) + f ( x , y ) + A x , y ) + f ( y , y ) , the inequality is

2,gf(x,Y)= f ( x , Y >+f(x,P,)d 2 ( f ( x , x)f(Y,Y))'12 which follows from Cauchy-Schwarz. The function x -+ ( f ( x , x))'12 therefore satisfies the conditions (I), (Ill), and (IV)of (Section 5.1); by (6.2.2), condition (11) of Section 5.1 is equivalent to the fact that the form f is nondegenerate. Therefore, when f is a nondegenerate positive hermitian form (also called a positive dejnite form), ( f ( x , x))'I2 is a norm on E. A preliilbert space is a vector space E with a given nondegenerate positive hermitian form on E; when no confusion arises, that form is written (x I y ) and its value is called the scalar product of x and y ; we always consider a prehilbert space E as a normed space, with the norm llxll = (x I x)''', and of course, such a space is always considered as a metric space for the corresponding distance Ijx - y ( / . With these notations, the Cauchy-Schwarz inequality is written

I(xlu)l d IIxIl

(6.2.4)

IIYII

and this proves, by (5.5.1), that for a real prehilbert space E, (x, y ) -+ (x 1 y ) is a continuous bilinear form on E x E (the argument of (5.5.1) can also be applied when E is a complex prehilbert space and proves again the continuity of (x, y ) (x I y ) , although this is not a bilinear form any more). We also have, as a particular case of (6.1 .I): --f

(6.2.5) (Pythagoras' theorem) onal vectors, IIX

In a prehilbert space E, i f x , y are orthog-

+ YIl2 = l/xll2+ llY/I2.

A n isomorphism of a prehilbert space E onto a prehilbert space E' is a linear bijection of E onto E' such that ( f ( x )If(y)) = (x Iy) for any pair of vectors x, y of E. It is clear that an isomorphism is a linear isonzetry of E onto E'. Let E be a prehilbert space; then, on any vector subspace F of E, the restriction of the scalar product is a positive nondegenerate hermitian form ; unless the contrary is stated, it is always that restriction which is meant when F is considered as a prehilbert space.

3 ORTHOGONAL PROJECTION ON A COMPLETE SUBSPACE

119

A Hilbert space is a prehilbert space which is coniplete. Any finite dimensional prehilbert space is a Hilbert space by (5.9.1); other examples of Hilbert spaces will be constructed in Section 6.4. If in example (6.1.4) we take a > 0, b 2 0 , c 3 0, it can be shown that the prehilbert space thus defined is not complete.

PROBLEMS

1. Prove the last statement in the case a = 1 , b = c = 0 (see Section 5.1, Problem 1). 2. (a) Let E be a real nornied space such that, for any two points ,Y, y of E, 1I.y

+ Yl12 + Ilx - Y1IZ

= 2(11x1I2

+ llvIl2).

Show that f ( x , y) = IIx i-V I -~ Ilxl/* ~ - /IyIl2 is a positive nondegenerate hermitian form on E. (In order to prove that f ( h x , y ) = h f ( x , y ) , use Problem I of Section 5.5.) (b) Let E be a complex nornied space, Eo the underlying real vector space. Suppose there exists on Eo a symmetric bilinear form f ( x ,y) such that f ( x , x) = l1x/l2for every x E E o . Show that there exists a herniitian form g(x, y ) on E such that f ( x , y ) = .jR(g(x, y ) ) , hence g(x, x) = 1/x1l2for x E E. (Using the first formula of Problem 1 of Section 6.1, prove that f ( i x , y ) = - f ( x , i-v).) (c) Let E be a complex normed space such that Ilx - Y1I2

+ Ilx + Y / I 2 < 2(11x1I2 + llvll’)

for any pair of points x , y of E. Show that there exists a nondegenerate positive hermitian fornif(x,y) on E such thatf(x, x) = XI/^. (Use (a) and (b)). 3. Letfbe a positive nondegenerate hermitian form. In order that both sides of (6.2.1) be equal, it is necessary and sufficient that x and y be linearly dependent. In order that both sides of (6.2.3) be equal, it is necessary and sufficient that x and ,y be linearly dependent, and, if both are # 0, that y = Ax, with real and > 0. 4. Let a, b, c, d be four points in a prehilbert space E. Show that

/(a- c ( / . / / b- d ( (< /la - bll . JJc- dll

5.

+ 116

- cii

. / / a- dll.

(Reduce the problem to the case a = 0, and consider in E the transformation x+x/llxl~’, defined for x # 0.)When are both sides of the inequality equal? be a finite sequence of points in a prehilbert space E. Show that Let ( x i ) ,

If /lxi - x,Jl > 2 for i # j , show that a ball containing all the x i has a radius >(2(n

-

1)jn)’”.

3. O R T H O G O N A L PROJECTION O N A COMPLETE SUBSPACE

(6.3.1) Let E be a prehilbert space, F a complete tvctor subspace of E (i.e. a Hilbert space). For any x E E, there is one and only one point y = P F ( x )E F

120

VI

HILBERT SPACES

such that I/x - J~II = d(x, F). Tlze point y = P,(x) is also the only point z E F such that x - z is orthogonal to F. The mapping x P,(x) of E onto F is linear, continuous, and of norm 1 if F # { O } ; its kernel F’ = PF1(0) is the subspace orthogonal to F, and E is the topological direct sum (see Section (5.4)) of F and F‘. Finally, F is the subspace orthogonal to F‘. --f

Let CI = d(x, F); by definition, there exists a sequence (y,) of points of F such that lim 1I.x - ynl/ = a ; we prove (y,,) is a Cauchy sequence. fl-+

co

Indeed, for any two points u, u of E, it follows from (6.1.1) that (6.3.1.1)

hence

112.4

llym

+

+ -

U(l2

1/24

PJ12= 2(llu112

+ 11z~112>

- yn/I2= 2(llx - ~ , ~ ,+l / IIx ~ - ~ , , l l ~-) 4llx - ! d Y m

+ y,)112.

But

+ y,,) E F, hence jlx - )(ym + y,,)lI2 > a 2 ;therefore, if no is such that for n 2 n o , ( / x- y , ( / 2< g2 + E , we have, form > no and n 3 n o , I/ym- Y,;;? < 4e, +(yJ,

which proves our contention. As F is complete, the sequence (y,) tends to a limit y E F, for which IJx- yll = d(x, F). Suppose y’ E F is also such that /Ix - y’ll = d(x, F); using again (6.3.11), we obtain / ( y- y’(I2= 4a2- 4 /Jx- ) ( y y’)1I2, and as j ( y y ’ ) E F, this implies (ly - y’1I2 < 0,i.e. y’ = y. Let now z # 0 be any point of F, and write that (lx - ( y Az)/l* > a2 for any real scalar A # 0; this, by (6.1 .I), gives

+

+

21B(x - y 1 z )

+

+ R 2 / ( 2 / ( 2> 0

and this would yield a contradiction if we had 9 ( x - y 1 z ) # 0, by a suitable choice of 1. Hence B(x - y 1 z ) = 0, and replacing z by iz (if E is a complex prehilbert space) shows that #(x - y I z ) = 0, hence (x - y I z ) = 0 in every case; in other words x - y is orthogonal to F. Let y’ E F be such that x -y’ is orthogonal to F; then, for any z # 0 in F, we have JIx- (y’ z)1I2 = /Jx- y‘/12 j/zIlz by Pythagoras’ theorem, and this proves that y’ = y by the previous characterization of y . This last characterization of y = P,(x) proves that P , is linear, for if x - y and x’- y’ are orthogonal to F, then Ax - Ay is orthogonal to F and so is (x + x’)- ( y + y’) = (x - y ) (x’ - y ’ ) ; as y y’ E F and Ay E F, this shows that y y’ = P F ( x x’)and Ay = P , ( ~ x ) .By Pythagoras’ theorem, we have

+

+

+

(6.3.1.2)

+

llX/12

=

+

IIpF(x)112

+

+ (Ix

- pF(x)l/2

and this proves that IlPF(x)l1< I/x/J,hence (5.5.1) P, is continuous and has norm d 1 ; but as P F ( x )= x for x E F, we have /lP,I/ = 1 if F is not reduced to 0. The definition of P, implies that F’ = PF’(0) consists of the vectors x orthogonal to F; as x = P,(x) + (X - PF(,y)) and x - P F ( x )E F’ for any x E E, we have E = F F’; moreover, if x E F n F‘, x is isotropic, hence

+

3 ORTHOGONAL PROJECTION ON A COMPLETE SUBSPACE

x

= 0, and

121

this shows that the sum F + F' is direct. Furthermore, the mapping

x -iP F ( x )being continuous, E is the topological direct sum of F and F' (5.4.2). Finally, if x E E is orthogonal to F', we have in particular (x I x - P F ( x ) )= 0 ; but we also have (PF(x) I x - PF(x))= 0, hence /Ix - PF(x)(I2= 0, i.e. x = PF(x) E F. Q.E.D. The linear mapping PF is called the orthogonal projection of E onto F, and its kernel F' the orthogonal supplement of F in E. Theorem (6.3.1) can be applied to any closed subspace F of a Hilbert space E (by (3.14.5)), or to any Jinite dimensional subspace F of a prehilbert space, by (5.9.1).

(6.3.2) Let E be a prehilbert space; then, for any a E E, x -i (x I a) is a continuous linear form of norm Ilall. Conversely, if E is a Hilbert space, for any continuous linear form u on E, there is a unique vector a E E such that u(x> = (x I a)for any x E E. ByCauchy-Schwarz,I(x I a)l < llall * IIxII, whichshows(by5.5.1))~+ (x la) is continuous and has a norm < llalj; on the other hand, if a # 0, then for x, = a / ~ ~ we a ~have ~ , (x, I a) = llall; as llxoII = 1, this shows the norm of x - i ( x l a ) is at least Ilall. Suppose now E is a Hilbert space; the existence of the vector a (=O) being obvious if u = 0, we can suppose u # 0. Then H = u-'(O) is a closed hyperplane in E; the orthogonal supplement H' of H is a one-dimensional subspace; let b # 0 be a point of H'. Then we have ( x I b) = 0 for any x E H. But any two equations of a hyperplane are proportional, hence there is a scalar L such that u(x) = L(x 16) = (xI a) with a = Xb (see Appendix) for all x E E. The uniqueness ofa follows from the fact that the form (x I y ) is nondegenerate.

PROBLEMS 1. Let B be the closed ball of center 0 and radius 1 in a prehilbert space E. Show that for each point x of the sphere of center 0 and radius I , there exists a unique hyperplane of support of B (Section 5.8, Problem 3) containing x . 2. Let E be a prehilbert space, A a compact subset of E, 6 its diameter. Show that there exist two points a , b of A such that /la- bll = 8 and that there are two parallel hyperplanesof support of A (Section 5.8, Problem 3)containingaand b respectively, and such that their distance is equal to 6. (Consider the ball of center a and radius 8 and apply the result of Problem 1 .)

3.

Let E be a Hilbert space, F a dense linear subspace of E, distinct from E (Section 5.9, Problem 2). Show that there exists in the prehilbert space F a closed hyperplane H such that there is no vector # 0 in F which is orthogonal to H.

VI

122

4.

HlLBERT SPACES

Let X be a set, E a vector subspace of Cx, on which is given a structure of complex Hilbert space. A mapping ( x , y ) K(x, y ) of X x X into C is called a reproducing kernel for E if it satisfies the two following conditions: ( I ) for every y E X, the function K(. , y ) : x --f K(x, y ) belongs to E; (2) for any function f~ E, and any y E X, f(y) = (fI K ( . , A). (a) Show that K is a mapping of positive fype of X x X into C, i.e., for any integer of points of X , the mapping n 3 1 and any finite sequence ( x i ) , --f

+x W x i , xi)&

((Ad, (pi))

pi

I. J

of C2" into C is a positiue hermitian form. This in particular implies K(x, x ) 3 0 for every x E X, K(y, x ) = K(x, y ) and IK(x, y)12 < K(x, x ) K ( y ,y ) for x, y in X. Show that f0rj.E E, one has lf(y)I < llfll . (K(y, y))"' for y E X. (b) Show that if (f.)is a sequence of functions of E which converges (for the Hilbert space structure) to f~ E, then, for every x E X , the sequence (f.(x)) converges to f ( x ) in C; the convergence is uniform in any subset of X where the function x + K(x, x ) is bounded. n a finite sequence of points of X, (al)lslsn a sequence of n com(c) Let ( x i ) l i 1 Bbe plex numbers. Suppose det(K(x, , x i ) ) # 0, so that the system of linear equations cj K(xi, x J )= al (1

< i < n) has a

J= 1

unique solution (ci). Show that among the func-

tions f~ E such that f(x1) = a , for 1 f i S n, the function fo

n

= cj J= I

K(. , x i ) has the

smallest norm. In particular, among all functions f~ E such that f ( x ) = 1 for a point E X where K(x, x ) # 0, the function K(. , x ) / K ( x ,x ) has the smallest norm. (d) If X is a topological space and if all the functionsf€ E are continu0u.s in X, then the functions K(.,x) (where x takes all values in X, or in a dense subset of X) form a total subset of E (show that there is no element h # 0 in E which is orthogonal to all the elements K(. , x ) ) . In particular, if X is a separable metric space, E is a separable Hilbert space. 5. (a) The notations being those of Problem 4, in order that there exist a reproducing kernel for E, a necessary and sufficient condition is that for every x E X, the linear form f + f ( x ) be continuous in E. The reproducing kernel is then unique. (b) Deduce from (a) that if there exists a reproducing kernel for E, there also exists a reproducing kernel for every closed vector subspace El of E. If K, is the reproducing kernel for E l , show that for every function f~ E, the function y --f (f1 K,(. ,y ) ) is the orthogonal projection o f f i n El. If E, is the orthogonal supplement of El and K2 the reproducing kernel for E, , then K I $- K 1 is the reproducing kernel for E. 6. Let X be a set, E a vector subspace of Cx, on which is given a structure of complex prehilbert space. In order that there should exist a Hilbert space c Cxcontaining E, such that the scalar product on E is the restriction of the scalar product on B, and that there exists a reproducing kernel for B, it is necessary and sufficient that E satisfy the two following conditions: ( I ) for every x f X, the linear form f + f ( x ) is continuous in E; (2) for any Cauchy sequence (f,)in E such that, for every x E X, lim f.(x) = 0, x

one has lim llfnll

n-m

= 0.

(To prove the conditions are sufficient, consider the subspace

f?

20'"

of Cx whose elements are the functionsffor which there exists a Cauchy sequence (fn) in E such that limfn(x) = f ( x ) for every x E X. Show that, for all Cauchy sequences (f.) n-r m

having that property, the number lim llfnll is the same, and if l i f l l is that number, this n-rm

defines on f a structure of normed space which is deduced from a structure of prehilbert

4 HILBERT SUM OF HILBERT SPACES

123

space which induces on E the given prehilbert structure; finally show that E is dense in 8 and that 8 is complete, hence a Hilbert space, and apply Problem 5(a) to 8.) 7. Let X be a set, f a mapping of X into a prehilbert space H; show that the mapping ( x , y ) + ( f ( x )1 f(y)) of X x X into C is of positive type (Problem 4(a)). 8 . Let X be a set, K a mapping of positive type of X x X into C (Problem 4(a)). (a) Let E be the set of mappings u : X -+ C such that there exists a real number a > 0 having the property that the mapping ( x , Y ) +aK(x, Y ) - 4x)UO

is of positive type; let m(u)be the smallest of all real numbersa 3 Ohavingthat property. Show that m(u) is also the smallest number c such that, for every finite sequence ( x i ) of elements of X, the inequality

holds for all complex numbers h, , p (use the Cauchy-Schwarz inequality). For every E X, show that Iu(x)12 < K(x, x)m(u). (b) Show that E is a vector subspace of Cx, that ( m ( ~ ) ) is ” ~a norm on E and that m(u u ) 4- m(u - u ) 2 2(m(u) m(u)).Conclude that there is a nondegenerate positive hermitian form g(u, u) on E x E such that g(u, u ) = m(u), and that for this form E is a Hilbert space; one writes g(u, u ) = (u I u). (Use Problem 2(c) of Section 6.2 to prove the existence of g ; to show that E is complete, use the last inequality proved in (a)). (c) For every x E X, show that the function K ( . , x ) belongs to E and that (K(. , x) I K ( . ,y ) ) = K(x, y ) for all ( x , y ) E X x X (use Cauchy-Schwarz inequality). Prove that if X is a topological space and if K is continuous in X x X, the mapping x + K(. , x) of X into E is continuous. (d) Deduce from (a) that the Hilbert space E defined in (b) has a reproducing kernel, and if F is the closed vector subspace of E generated by the functions K(. ,x), the reproducing kernel for F (Problem 5(b)) is equal to K. 9. Let E be a prehilbert space, N a finite dimensional vector subspace of E, M a vector subspace of E having infinite dimension, or finite dimension >dim(N). Show that there exists in M a point x # 0 such that llxll= d(x, N). (Consider the intersection of M and of the orthogonal supplement of N.) x

+

+

4. H I L B E R T SUM O F HILBERT SPACES

Let (En) be a sequence of Hilbert spaces; on each of the En, we write the scalar product as ( x , ) y n ) .Let E be the set of all sequences x = (xl, x 2 , . . . , x,, . . .) such that x, E En for each n , and the series ( I I x , ~ ~ ~ ) is convergent. We first define on E a structure of vector space: it is clear that if x = (x,) E E, then the sequence (Axl,.. . , Ax,,,. . .) also belong to E. On the other hand, if y = (y,) is a second sequence belonging to E, we observe that IIx, ynl12< 2(/1~,11~ liyn112) by (6.3.1.1), hence the series (llx, + y,I12) is convergent by (5.3.1), and therefore the sequence (xl + y , , ..., x, + y n r ...) belongs to E. We define x + y = (x,+y,,),

+

+

124

V

HILBERT SPACES

Ax = (Ax,), and the verification of the axioms of vector spaces is trivial. On the other hand, from the Cauchy-Schwarz inequality, we have

IIxnII *

I(xn IYn)I G

IIYnIt G

HIIXnII’

+ IIynII’).

Therefore, if x = (x,) and y = (y,) are in E, the series (of real or complex numbers) ((x, I y,)) is absolutely convergent. We define, for x = (x,) and

y = (y,) in E, the number (x 1 y ) =

00

,= 1

(x, 1 y,); it is immediately verified

that the mapping (x, y ) -.+ (x I y ) is a Hermitian form on E. Moreover we have (x Ix) =

1 llxn\1’, hence ( x l y ) is a positive nondegenerate

hermitian

n= 1

form and defines on E a structure of prehilbert space. We finally prove E is in fact a Hilbert space, in other words it is complete. Indeed, let (xc”’)), where x(”’)=(x$)), be a Cauchy sequence in E: this means that for any E > 0 there is an m, such that for p 2 m, and q >, m, , we have m

(6.4.1)

For each fixed n, this implies first

IIxp) - xP)1l2< E ,

( x ~ ~ ) ) ~ = ~is , a~ ,Cauchy ... sequence in

hence the sequence

En,and therefore converges to a

limit y , . From (6.4.1) we deduce that for any given N N

C IIxn(P) - xn(4)II2 < &

n= I

as soon as p and q are 2 m o , hence, from the continuity of the norm, we deduce that

N

n= 1


1 1 ~ : ~ ) - y,II’

integers N, we have

m “ .= .

for p 2 m,, and as this is true for all

IIxp)- ynllZG E .

I .

This proves first that the

sequence (xp’ - y,) belongs to E, hence y = (y,) also belongs to E, and - y11’ < E for p 2 m,, which ends the proof by showing we have (Idp) that the sequence (x(”’)) converges to y in E. We say that the Hilbert space E thus defined is the Hilbert sum of the sequence of Hilbert spaces (En). We observe that we can map each of the En into E by associating to each x, E En the sequence j,(x,) E E equal to (0, . . . , 0 , x, 0,. . .) (all terms 0 except the nth equal to x,); it is readily verified that j , is an isomorphism of En onto a (necessarily closed) subspace EA of E; j , is called the natural injection of En into E. From the definition of the scalar product in E, it follows that for m # n, any vector in EL is orthogonal to any vector in EL; furthermore, from the definition of the norm in E, it follows that for any x = (x,) E E, the series (j,(x,)) is convergent

4

in E, and x =

m

HILBERT SUM OF HILBERT SPACES

125

j,,(x,) (observe that the series (j,(x,)) is not absolutely con-

n= 1

vergent in general). This proves that the (algebraic) sum of the subspaces EA of E (which is obviously direct) is dense in E, in other words that the smallest closed vector subspace containing all the EA is E itself. Conversely: (6.4.2) Let F be a Hilbert space, (F,) a sequence of closed subspaces such that: (1) for m # n, any vector of F , is orthogonal to any vector of F,; (2) the algebraic sum H of the subspaces F , is dense in F . Then, i f E is the Hilbert sum of the F,, there is a unique isomorphism of F onto E which on each F, coincides with the natural injection j , of F, into E.

Let F; = j,,(F,,), and let h, be the mapping of FA onto F,, inverse to j , . Let G be the algebraic sum of the FA in E; that sum being direct, we can define a linear mapping h of G into F by the condition that it coincides with h, on each FA. I claim that h is an isomorphism of G onto the prehilbert space H (which, incidentally, will prove that the (algebraic) sum of the F, is direct in F ) ; from the definition of the scalar product in E, we have to check that

for Xk E Fk,yk E F k ; but by assumption (x, I yk) = 0 if h # k, and the result follows from the fact that each j k is an isomorphism. There is now a unique continuous extension T; of h which is a linear mapping of = E into = F, by (5.5.4); the principle of extension of identities (3.1 5.2) and the continuity of the scalar product show that h is an isomorphism of E onto a subspace of F, which, being complete and dense, must be F itself; the inverse of h satisfies the conditions of (6.4.2). Its uniqueness follows from the fact that it is completely determined in G and continuous in E (3.15.2). Under the condition of (6.4.2), the Hilbert space F is often identified with the Hilbert sum of its subspaces F , .

e

Remark (6.4.3)

We can also prove (6.4.2) by establishing first that the sum of the F, n

is direct; indeed, if E x i = 0 with x i €Fi (1 < i < n) we also have i = l.

= 0 for any j

< n, and as (xjIxi) = 0 for i # j , this boils down

126

VI HILBERT SPACES

to llxil12 = 0, hence xi = 0 for 1 < j < n. Then we define the inverse mapping g of h by the condition that it coincides with j,, on each F,,: we at once verify, as above, that g is an isomorphism of H onto G, and then (5.5.4) is applied in the same way. We observe that this argument still applies when F is a prehilbert space and the F, are complete subspaces of F; it proves the existence of an isomorphism of F onto a dense subspace of the Hilbert sum E of the F,,, which coincides withj,, on each F, .

PROBLEM

Let X be a set, E l , Ez two vector subspaces of Cx, possessing reproducing kernels K,, K2 (Section 6.3, Problem 4). Let E = El E2 c Cx,and let F be the Hilbert sum of the Hilbert spaces El , Ez;the kernel of the surjective mapping u : (fl, fz)-fl +fz of F into E is the subspace N of F consisting of the pairs (f,-f) where YE El n EZ. Show that N is closed in F; if H is the orthogonal supplement of N in F, the restriction u of u to H is a bijection of H onto E; transporting by u the Hilbert structure of H, E is defined as a Hilbert space. Show that for that Hilbert space structure, E has a reproducing kernel equal to K1 Kz; the norm llfll in E is equal to inf(llflII: llfzll~)l~zwhen (fl,f2) takes all values in F such thatf=fl +fz(llfl Illand llf~ [I2 being the norms in El and E2 ,respectively).

+

+

+

5. ORTHONORMAL SYSTEMS

If (with the notations of Section 6.4) we take for each E,, a one-dimensional space (identified to the field of scalars with the scalar product (5 I q) = (fj), the Hilbert sum yields an example of an infinite dimensional Hilbert space E, which is usually written l2 (with index R or C to indicate if necessary what the scalars are); the space I: (resp. 1;) is therefore the space of all gi

sequences x = (5,) of real (resp. complex) numbers, such that convergent, with the scalar product ( x I y ) =

c m

l{,,I2 is n= 1

{, ij,

.

n=l

In 12, let en be the sequence having all its terms equal to 0 except the nth term equal to 1; we then have (em1 en)= 0 for m # n, and IJe,,JI= 1 for each n, and we have seen in Section 6.4 that for every x = (t,) in f 2 , we can write x =

c <,en, the series being convergent in 12. We observe that this m

n= 1

shows the sequence (en)is total in 12, hence (5.10.1) 1’ is separable. Let us now consider an arbitrary prehilbert space F; we say that a (finite or infinite) sequence (a,) in F is an orthogonal system if (a, I a,) = 0 for m # n and a,, # 0 for every n ; we say that (a,) is an orthonormal system

5 ORTHONORMAL SYSTEMS

127

if in addition lla,,II = 1 for each n. From any orthogonal system (a,,) we deduce at once an orthonormal system by “normalizing” (a,,), i.e. considering the sequence of the b,, = a,,/\lanl\.We have just seen an example of an orthonormal system in 1’; another fundamental example is the following: (6.5.1) Let I be the interval [- 1, 11 of R, and let F = VJI) be the vector space of all continuous complex-valued functions defined on I. We define on F a scalar product by

J -1

(the fact that this is a nondegenerate positive hermitian form is readily verified). For each positive or negative integer n, let

q,,(t) = ennit. It is readily verified that (q,,/,/2)is an orthonormal system in F, called the trigonometric system. Let now (a,) be an arbitrary orthonormal system in a Hilbert space F; for each x E F, we say that the number c,,(x) = ( x I a,,) is the nth coefficient (ornth coordinate) ofx withrespect to the system (a,,) (“nth Fourier coefficient ” of x for the system (6.5.1)). (6.5.2) In a Hilbert space F, let (a,,) be an orthonormal system, V the closed subspace of F generated by the a,, . Then,for any x E F; 1.

the series

m

1 I(x I a,,)1’

is convergent, and we have

n= 1

and

2.

the series of general term (x I a,)a,, is convergent in F and we have m

1 (x I an)% =

n= 1

P”(X).

Conversely, let (A,,) be a sequence of scalars such that

m

1 1A,,I2 is convergent.

n=1

Then, there exists a unique vector y E V such that ( y I a,,) = A,, for every n ; any other vector x E F such that (X I a,,) = Anfor every n is such that x = y + z , with z orthogonal to V, and conversley.

128

VI HILBERT SPACES

For any x E F, we can write x = Pv(x) + 2 , with z orthogonal t o V (6.3.1) and we have therefore (x I a,) = (P,(x) I a,). To prove the theorem, we may therefore assume V = F; but then, the one-dimensional subspaces F, generated by the vectors a, satisfy the assumptions of (6.4.2), and the results are mere restatements of (6.4.2) for that particular case (taking into account the definition of a Hilbert sum). The most interesting case is that in which V = F, i.e., the orthogonal system (a,) is total. It is then called a Hilbert basis for F; (c,) is such a basis for 12. It will be proved in (7.4.3)that the trigonometric system (6.5.1) is total. For a Hilbert space F and a total orthonormal system (a,), we can replace everywhere P, by the identity in (6.5.2); the relations n

n=

I

are then called Parseral’s identities. It follows at once from (6.5.2) that these identities represent not only necessary but su#cient conditions for (a,) to be a total system in a Hilbert space. (6.5.3) It1 a Hilbert space F, a necessary arid sufJicient condition for an orthogonal system (a,) to be total is that the relations (x I a,) = 0 for every n imply .x = 0. Indeed, by (6.5.2) this means that the relation Pv(x) = 0 implies x = 0, and this is equivalent to the relation V = F, since P,(x - P v ( x ) )= 0.

Remark (6.5.4) Suppose E is a prehilbert space and the orthonormal system (a,) in E is total. Then the results ( I ) and (2) of (6.5.2) are still valid, with Pv(x) replaced by x; this follows by the same argument as in (6.5.2), using the Remark (6.4.3).

PROBLEMS

1. Let E be a Hilbert space with a Hilbert basis (e,,),,zl.Let A be the subset of E

consisting of the linear combinations x

n

= k= 1

(n arbitrary).

k= I

6 ORTHONORMALIZATION

(a> Show that the closure where An > 0, the series

-

A is the set of all the sums of the series

I

C A,, is convergent

129

"= 1

and has a sum equal to I .

"=I

(b) Prove that the diameter of A is equal to d2 but that there is no pair of points A such that /la- b/l = 42 (compare to Section 6.3, Problem 2). Let E be a Hilbert space with a Hilbert basis Let a, = eZn and

a, h of

b, = ezn

1 ++ 1 e z n f lfor every n > 0; letA (resp. B) be the closed vector subspace of E I1

generated by the a. (resp. b"). Show that: (a) A n B = { O } , hence the sum A B is direct (algebraically). (b) The direct sum A B is not a topological direct sum (consider in that subspace the sequence of points h, - a, and apply (5.4.2.)). (c) The subspace A B of E is dense but not closed in E (show that the point m

+

+ +

(b, - an)does not belong to A

+ B).

"=O

Show that the Banach space 9 ( P ; 1 2 ) can be identified with the space of double

sequences U = (a,,,,) such that: (I) the series m

( 2 ) sup n

m

nr= 0

is convergent for every n:

la,,,nlzis finite. The norm is then equal to IlUIj = sup

"

nt=O

method as in Section 5.7, Problem 2(b)). (a) Let u be a continuous linear mapping of show that the series

c m

fl=O

and that their sums are on E and use (6.3.2).)

/cz,,,12 and

<

c m

/a,,,,Jz are

(Observe that x

into itself, and let u(e,,) =

x

convergent for all values of m and n,

--z

( ~ ( xI e,") ) is a continuous linear form m

"=O

la,nnlz <1

for all values of m and

11,

1 a,m,e,;

lll=O

(b) Give an example of a double sequence (a,",J such that m

(same

m

P

Wt=O

lIi(l/'.

imYo)

112

(a,,,('

<

(anln/* 1

and

but such that there is no continuous linear

m=0

mapping it of P into itself satisfying the relations (u(e,) I e,,,)= a,,,, for all pairs (m, n). (If V is a subspace of I z generated by the vectors en with n E H, where H is a set of p integers, show that there is a linear mapping up of V into itself such that (//,,(en)I e",)= I/t'p for all indices m, 11 of the set H, but l ! i t p l l > v'p.)

6. ORTH 0 NORMALl ZATl O N

(6.6.1) Let E be a separable preliilbert space, (6,) a total sequence of linearly independent cectors in E (see (5.10.1)), and let V, be the n-dimensional subspace of E generated by b,, . . . , 6,. Then if we define c, = b, - Pv,-,(b,), (c,) is a total orthogonal system, such that cl, . . . , c, generate V, for each n.

We use induction on n, assuming that cl, . . . , c , - ~ is an orthogonal system generating V n - l ; then, by definition of Pv,-l (6.3.1),c, is orthogonal

130

VI

HILBERT SPACES

to V,,-l, which proves that (ciIc j ) = 0 for 1 Q i < j < n ; moreover, as b, # V,-l by assumption, c, # 0, hence e l , . , . , c, is an orthogonal system; moreover, b,, - c,, E V,,...l, hence c l , . , . , c, generate the same subspace as the union of V,-l and {b,,},i.e. V , . This completes the proof.

If we normalize the system (c,,), by putting a, = c,,/~lc,,Il, the system (a,) is said to be deduced from (h,,) by the orthonormalization process. For instance, in the space F = %'Jl) considered in (6.5.1), the sequence (t") is total (as will be proved in (7.4.1)) and obviously consists of linearly independent vectors. If we denote by (Q,) the orthonormal system deduced from (t") by orthonormalization, it is clear that Q,,(t) = a,, t" + * . * , polynomial of degree n (with a, # 0) with real coefficients; the Q, are (up to a constant factor) the Legendre polynomials (see Section 8.14, Problem 1). (6.6.2) Any separable prehilbert space (resp. Hilbert space) is isomorphic to a dense subspace of I' (resp. to 1'). As there exists in a separable prehilbert space a total orthonormal system by (6.6.1), the result follows at once from (6.5.2).

PROBLEMS

1. Let E be a separable noncomplete prehilbert space. Show that there exists in E an orthonormal system which is not total, but which is not properly contained in any orthonormal system (imbed E as a dense subspace of a Hilbert space, and use problem 3 of Section 6.3). 2. Let E be a n infinite dimensional separable Hilbert space, V a closed vector subspace of E. Show that if V is infinite dimensional, there exists an isometry of E onto V (write E as the direct sum of V and its orthogonal supplement V', and take Hilbert bases in V and V'. 3. Let (xi)l<, C n be a finite sequence of points in a prehilbert space E. The Gram determinant of that sequence is the determinant G ( x l ,xz , ... ,x.) = det((xi 1 x i ) ) . (a) Show that G ( x , ,. . . , x.) 2 0 and that G(x1,. . , x,) = 0 if and only if the x I are linearly dependent. (Consider a Hilbert basis of the sub space generated by the xi, and express the x1 as linear combinations of that basis.) (b) Suppose the xi are linearly independent, and let V be the n-dimensional subspace which they generate. Show that the distance of a point x to V is equal to

.

(G(x,X I , * 4.

I . ,

xn)/G(xi,. .. ,x,))"~

(find the projection of x on V, writing it as a linear combination of the xi). Let M be a compact subset of a Hilbert space E; if El is the smallest closed vector subspace of E containing M, show that El is separable.

6

ORTHONORMALIZATION

131

5. Let E c Cx a separable Hilbert space having a reproducing kernel (Section 6.3, Problem 4). (a) Let (At) be a Hilbert basis for E. Show that for every (x, y ) E X x X one has K(x, y ) = c f . ( x ) f , o , where the series converges in C. For any function g E E, if c, = (g

Ifn),

I!

one has, for every x E X, g(x)

=c

cnfn(x), where

the series converges in C;

n

in addition, that series is uniformly convergent in every subset of X where K(x, x ) is bounded. (b) Conversely, let (fn) be a sequence of complex functions defined in a set X, such Ifn(x)12 < ca. For every sequence (c,) E I;, the series that for every x E X,

+

cnf.(x) is then convergent in

C for every x

E

X. The functions which are the sums of

such series form a vector subspace E c Cx,on which one defines a structure of separable Hilbert space by taking as scalar product, for u c,f,, u d. f,, the number

=c n

(u I u ) =

c. n

=c n

-

d, . This space has a reproducing kernel K(x, y ) =cfn(x)J,(y). "

CHAPTER VII

SPACES OF CONTINUOUS FUNCTIONS

Spaces of continuous functions are second only to Hilbert spaces as to their importance in functional analysis. Their definition makes it possible to give a much more intuitive meaning to the classical notion of uniform convergence. The most important results of the chapter are: 1. the StoneWeierstrass approximation theorem (7.3.1), which is a very powerful tool for the proof of general results on continuous functions, by the device which consists in proving these results first for functions of a special type, and then extending them to all continuous functions by a density argument; 2. the Ascoli theorem (7.5.7),which lies at the root of most proofs of compactness in function spaces, and, together with (7.5.6), gives the motivation for the introduction of the concept of equicontinuity. The last section of Chapter VII introduces, as a useful technical tool in the development of calculus, a category of functions which are classically described as “functions with discontinuities of the first kind”; in an effort towards a more concise expression, and to avoid one more use of the overworked term regular,” the author has tentatively introduced the neologism “ regulated functions (corresponding to the French fonctions reglees ”), which he hopes will not sound too barbaric to English-speaking readers. “

”

“

1. SPACES O F B O U N D E D F U N C T I O N S

Let A be any set, F a real (resp. complex) normed space; a mapping f o f A into F is hounded iff(A) is bounded in F, or equivalently if sup \lf(t)ll fEA

is finite. The set BIF(A)of all bounded mappings of A into F is a real (resp. g(t)ll d llf(r)ll Ilg(t)II. Moreover, on complex) vector space, since Ilf(r) this space,

+

132

+

1 SPACES OF BOUNDED FUNCTIONS

133

(7.1 .I) is a norm, as can be trivially verified. If F has finite dimension, and (aJl is a basis for F such that Jlaill= 1, any mapping of A into F can be written in one and only one way (7.1.1.1)

t -+f(t) = f * ( t ) a , + * * +.LSt)a,

and f is bounded if and only if the scalar mappings fi ( 1 d i d n ) are bounded. Moreover the norm of the mapping f - + f i ( t ) a iis llfill * /lai/J= /Ifill (the norm of fibeing taken in @R(A), resp. %,-(A)). From (5.9.1), (5.4.2), and (5.5.1) it follows that there is a constant c such that for each t E A , Ifi(t)l d c . Ilf(t)II, hence llfill d c * llfll. Let Li be the subspace of BF(A) consisting of all bounded mappings of the form t - t f ( t ) a i ( f scalar). Then, using again (5.4.2) and (5.5.1), the preceding remarks prove that (7.1.2) If F has finite dimension, tlien g,(A) is the topological direct sum of the L i , each of which is isonwtric to g R ( A ) (resp. 9Ic(A)).

In particular, if we consider the real normed vector space underlying &?,-(A), we see that it is the topological direct sum BR(A) + igR(A). (7.1.3) I f F is a Banaclt space, $#F(A)is a Banacli space. Let ( h )be a Cauchy sequence in BF(A); this means that for any E > 0 there is no such that llfm --f v I l < E for m 3 n o , n 3 n o . From (7.1.1) it follows that for any t E A we have Ilf,(t) -.h(f)ll< E for m 3 n o , n 3 n o , hence, as F is complete, the sequence ( f , ( t ) )converges to an element g ( t ) E F. Furthermore we have, by the principle of extension of inequalities, llf,,(t) - g(t)ll < c: for any t E A and all m 3 n o . From this we first deduce that Ilg(t)I[d (1 i,,ll + c for all t E A, hence g is bounded. Moreover we have IIj,, - 911 < E for all m 3 n o , and this means the sequence (f,)converges to g in the space %F(A). I n general, if (f,) is a sequence of mappings of A into a metric space F, we say that the sequence (f,) conrerges simp1.v in A to a mapping g of A into F if, for each t € A , the sequence ( f , ( t ) ) converges in F to g ( t ) ; we say that (f,)coniwges uniformly in A to g if the sequence of numbers (sup cl(f,(t), g ( t ) ) ) tends to 0. It is clear that uniform convergence implies isA

simple convergence; the converse is not true. If F is a normed space, convergence of a sequence o f elements of gF(A) therefore means, by definition,

134

VII

SPACES OF CONTINUOUS FUNCTIONS

uniform convergence of the sequence in A. Similarly, we say that a series (u,) which conuerges in BF(A) to a sum s is uniformly convergent in A to the sum s. If F is a Banach space, it follows from (7.1.3) that in order that a series (u,) in BF(A) be uniformly convergent, a necessary and sufficient condition is that, for any E > 0, there exist an integer no such that, for n 2 n o , p 2 0 and any t E A , we have Ilu,(t)

+ u,+,(O +

* *

'

+ u,+,(OlI

G

E.

From (7.1.3) and (5.3.2) it follows that if F is a Banach space and if a series (u,) of bounded functions is such that the series (llu,,II) converges in R, then the series (u,) is uniformly convergent; moreover, for each ~ E A since , IIu,,(t)ll d IIu,(I the series (u,(t)) is absolutely convergent in F. However, these two properties do not imply that the series (Ilu,ll) is convergent; to avoid misunderstandings, we therefore say that the series (u,) is normally convergent in BF(A) if the series (IIu,ll) converges. We define similarly a normally summable family ( u J AE L in BF(A) (L denumerable, cf. Section 5.3).

PROBLEMS

+

1. In the space .@R(R),let I!, be the function equal to l/n for n < t < w I , to 0 for other values of t . Show that the series (u,) is uniformly and commutatively convergent (Section 5.3, Problem 4) and that for every f t R, the series (u,(f)) is absolutely convergent, but that (u,) is not normally convergent. 2. Let A be any set; show that the mapping u+sup u ( r ) of dR(A) into R is continuous. 1EA

3.

Let E be a metric space, F a normed space; show that the set of all mappingsfc- .?A,(E) whose oscillation (Section 3.14) at every point of E is at most equal to a given number c( > 0, is closed in the space .H, (E).

2. SPACES O F B O U N D E D C O N T I N U O U S F U N C T I O N S

Let E now be a metric space; we denote by %F(E) the vector space of all continuous mappings of E into the normed space F, by %',"(E) the set of all bounded continuous mappings of E into F. We note that if E is compact, %?F(E) = %F(E) by (3.17.10). I n general we have %F(E) = %?F(E)n BF(E). We will consider %?F(E)as a normed subspace of gF(E), unless the contrary is explicitly stated. If F is finite dimensional, in the decomposition (7.1.2.1) f is continuous if and only if each of the fiis continuous (see (3.20.4) and (5.4.2)). The remarks preceding (7.1.2) then show that in such a case, %'F(E) is a topological direct sum of a finite number of subspaces, each of which

2 SPACES OF BOUNDED CONTINUOUS FUNCTIONS

135

is isometric to Wg(E) (resp %‘,?(E)). I n particular, the real normed space underlying V,‘(E) is the topological direct sum %‘g(E)+ iV:(E).

(7.2.1) The subspace W;(E) is closed in BF(E); in other ~1ords,a uniform limit of bounded continuous functions is continuous. Indeed, let (f,) be a sequence of bounded continuous mappings of E into F, which converges to g in g3,(E); for any E > 0, there is therefore an integer no such that ilf, - 911 < 4 3 for n 2 n o . For any to E E, let V be a neighborhood of to such that ~l.f,,(t) -fflo(to)l\d 4 3 for any t E V. Then, as Ilf,,(t) - g(t)I/ d 4 3 for any t E E, we have /lg(t)- g(t,)ll < E for any t E V, which proves the continuity of g. Well-known examples (e.g. the functions x -+ x” in [0, 13) show that a limit of a simply convergent sequence of continuous functions need not be continuous. On the other hand, examples are easily given of sequences of continuous functions which converge nonuniformly to a continuous function (see Problem 2). However (see also (7.5.6)):

(7.2.2) (Dini’s theorem) Let E be a compact metric space. Ifa n increasing (resp. decreasing) sequence (f f l )of real-aalued continuous functions converges simply to a continuous function g, it conwrges uniformly to g. Suppose the sequence is increasing. For each E > 0 and each t E E , there is an index n(t) such that for rn 2 n ( t ) , g(t) -f,(t) < e/3. As g and f f l ( r ) are continuous, there is a neighborhood V ( t ) oft such that the relation t’ E V ( t ) implies Ig(t) - g(t’)l < 4 3 and 1fnct,(t) - f&,(t’)l d 4 3 ; hence, for any t‘ E V(t) we have g(r’) -Lcr,(t’) d E . Take now a finite number of points r i in E such that the V(ti) cover E, and let no be the largest of the integers n(t,). Then for any t E E, t belongs to one of the V(ti), hence, for n 2 no , d t ) -L(t) -f,,(t) d A t ) -fncr,)(t) 6 8 . Q.E.D.

PROBLEMS

1. Let E be a metric space, F a normed space, (u,,) a sequence of bounded continuous mappings of E into F which converges simply in E to a bounded function u. (a) In order that u be continuous at a point x, E E, it is necessary and sufficient that for any E > 0 and any integer m, there exist a neighborhood V of x , and an index n > m such that Ilu(x) - u.(x)II < E for every x E V. (b) Suppose in addition E is compact. Then, in order that u be continuous in E, i t is necessary and sufficient that for any E > 0 and any integer m,there exist a finite number

136

VII SPACES OF CONTINUOUS FUNCTIONS

of indices n, > m such that, for every x E E, there is at least one index i for which Ilv(.x) - u,,(x>Il < E (use (a) and the Borel-Lebesgue axiom).

2.

For any integer ti > 0, let g. be the continuous function defined in R by the conditions that g,,(t) = 0 for t < O and I 3 2 / n , g,,(I/n)= 1 , and g,(t) has the form orf p (with suitable constants a , p) in each of the intervals [O, l / n ] and [l/n, 2 / n ] .The sequence (gn) converges simply to 0 in R, but the convergence is not uniform in any interval containing t = 0. Let m i r , , be a bijection of N onto the set Q of rational numbers, and let

+

OL.

fn(t) =

C 2-mg.(t - r,,,). The functions f, are continuous (7.2.1 ), and the sequence (A) lfl=O

3.

4.

5. 6.

7.

8.

converges simply to 0 in R, but the convergence is not uniform in any interval of R . Let 1 be a compact interval of R,and ( f n ) a sequence of monotone real functions defined in I, which converge simply in I to a continuous functionf. Show that f i s monotone, and that the sequence (f.)converges uniformly to f i n I. Let E be a metric space, F a Banach space, A a dense subset of E. Let (fJ be a sequence of bounded continuous mappings of E into F such that the restrictions of the functions fn to A form a uniformly convergent sequence; show that ( f n ) is uniformly convergent in E. Let E be a metric space, F a normed space. Show that the mapping (x, N) + u(x) of E x %F(E)into F is continuous. Let E, E' be two metric spaces, F a normed space. For each mappingfof E x E'into F and each y E E', let f, be the mapping x - f ( x , y ) of E into F. (a) Show that if f i s bounded, if each f, is continuous in E and if the mapping y +A, of E' into XF(E) is continuous, thenfis continuous. Prove the converse if in addition E is compact (use Problem 3(a) in Section 3.20). (b) Take E = E' = F = R,and let f ( x , y ) = sin x y , which is continuous and bounded in E x E'; show that the mapping y - f , of E' into '6F(E) is not continuous at any point of E'. (c) Suppose both E and E' are compact, and for any f E KF(Ex E ) , let be the mapping y -f, of E' into KF(E); show that the mapping f-3 is a linear isometry of PF(E x E') onto % ' C ~ ~ ( ~ ) ( E ' ) . Let E be a metric space, F a normed space. For each bounded continuous mappingf of E into F, let G ( f )be the graph of f i n the space E x F. (a) Show that f - G ( f ) is a uniformly continuous injective mapping of the normed space %F(E) into the space a(E x F) of closed sets in E x F, which is made into a metric space by the Hausdorff distance (see Section 3.16, Problem 3). Let r be the image of VF(E) by the mappingf-- G ( f ) . (b) Show that if E is compact, the inverse mapping G - ' of I' onto %,"(E) is continuous (give an indirect proof). (c) Show that if E = 10, 13 and F = R, G - ' is not uniformly continuous. Let E be a metric space with a bounded distance cl. For each x E E let d, be the bounded continuous mapping y d(x, y ) of E into R. Show that x r!, is an isometry of E onto a subspace of the Banach space %g(E). --f

--f

3. T H E STONE-WEIERSTRASS A P P R O X I M A T I O N T H E O R E M

For any metric space E, the vector space %'g(E) (resp.%?E(E)) is an algebra over the real (resp.complex) field; from (7.1.1) it follows that we have in that algebra llfgll < llfll * Ilgll, hence, by (5.5.1), the bilinear mapping (5 g) -fg

3 THE STONE-WEIERSTRASS APPROXIMATION THEOREM

137

is continuous. From that remark, it easily follows that for any subalgebra A of Vg(E) (resp. V,?(E)), the closure m of A in Vg(E) (resp. VF(E)) is again a subalgebra (see the proof of (5.4.1)). We say that a subset A of BR(E) (resp. B,-(E)) separates points of E if for any pair of distinct points x , y in E, there is a function f E A such that f ( x ># f ( Y ) . (7.3.1) (Stone-Weierstrass theorem) Let E be a compact metric space. If a subalgebra A of 'eR(E) contains [he constant functions and separates

points of E, A is dense in the Banach space VR(E).

I n other words, if S is a subset of WR(E) which separates points, for any continuous real-valued function f on E, there is a sequence (g,) of functions converging uniformly t of, such that each g, can be expressed as a polynomial in the functions of S, with real coefficients. The proof is divided in several steps. (7.3.1 .I) There exists a sequence of real polynomials (u,) which in the interval [0, 13 is increasing and concerges uniformly to J t

.

Define u, by induction, taking u1 = 0, and putting (7.3.1.2)

u,+l(t) = u,(t)

+ +(t - u,2(t))

for n 2 1.

We prove by induction that u , + ~2 u, and u,(t) < JTin [0, 11. From (7.3.1.2), we see the first result follows from the second. On the other hand Jt - u , + , ( t ) = J t -

u,(t) - S ( t

= (Jt- u,(l))(l

- U,2(t))

- +(&+

u,(t>>)

and from un(t) < ,/r we deduce f ( J t + u,(t)) < Jt< 1. For each t E [0, 11, the sequence (u,(t)) is thuj increasing and bounded, hence converges to a limit r ( t ) (4.2.1); but (7.3.1.2) yields f - v 2 ( t ) = 0 and as C(f) 2 0, o(t) = Jt. As c is continuous and the sequence (u,) is increasing, Dini's theorem (7.2.2) proves that (u,) converges uniformly to D.

(7.3.1.3)

For any function f

E

A, If 1 belongs to the closure

A of A on VR(E).

Let a = (1 f (1. By (7.3.1.1), the sequence of functions u , ( f 2 / a 2 ) , which belong to A (by definition of an algebra), converges uniformly to(fz/a2)'/2 = If I/a in E.

138

VII SPACES OF CONTINUOUS FUNCTIONS

(7.3.1.4) to A.

For any pair of functions f, g in A, inf(f, g) and sup(f, g) belong

For we can write sup(f, g) = f( f + g + 1f - gl) and inf(f, g) = f ( f + g - If - gl); the result therefore follows from (7.3.1.3) applied to

the algebra

(7.3.1.5) c1,

A.

For any pair of distinct points x, y in E and any pair of real numbers

p, there is a function f E A such that f ( x ) = u, f ( y ) = 8.

By assumption, there is a function g E A such that g(x) # g(y). As A contains the constant functions, take f = a + ( p - a)(g - y)/(S - y), where y = g(x),& = d Y ) . (7.3.1.6) For any function f E %,(E), any point x E E, and any E > 0 , there is a function g E A such that g(x) =f(x) and g(y)
For any point z E E, let h, be a function of A such that h,(x) =f ( x ) and h,(z) < f ( z ) + ~ / 2 the ; existence of such a function is obvious for z = x and follows from (7.3.1.5) for z # x . There exists a neighborhood V ( z ) of z such that for y E V(z),h,(y)
+

since every y

EE

i

belongs to some V(zi).

Let f be any function of %‘R(E);for any E > O and for each X E E , let A be such that g,(x) =f(x) and gx(y)
i

+

belongs to A and is such that, for any y E E, f ( y ) - E < cp(y) < f ( y ) E (since every y E E belongs to some U(x,)); in other words I(f - cpI( < E , and this shows that f belongs to the closure of A, i.e. to A itself. The corresponding theorem for VC(E)is false (see Chapter IX); there is only the weaker result:

4 APPLICATIONS

139

(7.3.2) Let E be a compact metric space. I f a subalgebra A of Vc(E) contains the constant functions, separates points of E, and is such that for each f E A the conjugate function J also belongs to A, then A is dense in Vc(E).

+

We remark that for any f E A, 9?f = +(f f ) and Yf= (f- f ) / 2 i also belong to A; hence, if A, is the (real) subalgebra of A consisting of realvalued functions, it follows at once from the definition that A, separates points of E and contains the (real) constant functions. Therefore A, is dense in %R(E), and the density of A in Wc(E) = WR(E) iVR(E) follows at once, since A = A, iA, .

+

+

4. APPLICATIONS

In the Stone-Weierstrass theorem, take for E any compact subset of R", and for A the algebra of the restrictions to E of the polynomials in the n coordinates. The separation condition is satisfied, since for two distinct points of E, at least one of the coordinates has distinct values. Hence we have the original Weierstrass approximation theoEm : (7.4.1) Any real-valued continuous function on a compact subset E of R" is the limit of a sequence of polynomials which converges uniformly in E.

Take now for E the unit circle x2 + y 2 = 1 in R2, parametrized by the angle nt, so that continuous functions on E can be identified with continuous functions on R having the period 2 (see Chapter IX). Take for A the (complex) algebra generated by the constants and the functions eni' and e-nir; it is immediate that the elements of A are the trigonometric polynomials

N

n=-N

c, ennit.

As the function enirseparates the points of E, all the conditions of (7.3.2) are satisfied, hence :

(7.4.2) Any continuous complex-valued function on R, which is periodic of period 2, is the limit of a sequence of trigonometric polynomials, which converges uniformly in R.

This last result enables us to give a proof of the following fact, which was announced in Section 6.5:

140

VII SPACES OF CONTINUOUS FUNCTIONS

(7.4.3) The trigonometric system is total in the preliilbert space F = %‘c(I) (as defined in (6.5.1); note that here we do not put on %,(I) the norm (7.1 .I)).

Indeed, for any function f E %‘,-(I) and any integer n > 0, let g be the function equal to f for - 1 -t- l / n < t < I , equal to f(1) for t = -1, and linear between - 1 and - 1 + l / n ; then f ( t ) - g(t) = 0 if t 2 - 1 l/n, and IJ(t) - g(t)l < 4lIfl1, for the other values o f t (we write 11.. /Im for the norm defined by (7.1.1) and 11 . . (I2 for the prehilbert norm). Therefore, we have

+

j If(0 I

llf-

911: =

- d9I2d t

< 16llfllZ,/n;

-1

in other words, IIf-gl12 is arbitrarily small. As g is continuous and can be extended by periodicity since g(1) = g( - I), there is, by (7.4.2), a trigonometric polynomial h such that 119 - / i l l z < J 2 ( ( g - / ~ is ~ arbitrarily ~ m small, and this ends the proof. (7.4.4) If E is a compact metric space, tlie spaces %‘R(E) and %‘?,(E)are separable.

As gC(E) is the topological direct sum of gR(E) and iVR(E), we need only give the proof for VR(E). Let (U,) be a denumerable basis for the topology of E (3.1 6.2),and let g,(t) = d(t, E - U,). The monomials g;‘ * . g: in the g,, also form a denumerable set (/I,) (by (1.9.3) and (1.9.4)), and the vector space A generated by the / I , is the subalgebra of WR(E) generated by the 9,.If we prove that A is dense in gR(E),our proof will be complete (5.1 0.1); but we only have to apply the Stone-Weierstrass theorem, and therefore check that the family (g,) separates points of E. But if x # y , there is a U, such that x E U,, y 4 U,, hence by definition g,(x) # 0, gn(y)= 0. Q.E.D.

PROBLEMS

1. Let E, F be two compact metric spaces, f a continuous mapping of E x F into R. Show that for any E > 0 there exists a finite system ( u I ) l l of continuous mappings of E into R and a finite system (uJl 6 , C n of continuous mappings of F into R such that,

for any (x, y ) E E

y

F, lf’(x, y ) -

”

I=I

u,(x)a,(,v)l C

E.

(Apply the Stone-Weierstrass

theorem to the algebra generated by the continuous mappings (x,J]) + I I ( X ) (x,y ) + u ( y ) , where N E KR(E) and v E CR(F).)

and

5 2.

EQUICONTINUOUS SETS

141

Let n + r,, be a bijection of N onto the set of rational numbers in the interval [0, I ] = I. Define by induction a sequence (1.) of closed intervals contained in I, such that: ( I ) the center of I, is v,, , where k , is the smallest index p such that Y,, is not in the union of the intervals I with h < n ; (2) the length of 1. is < 1/4", and 1. does not meet any of the I,, with h < n. In the product space I x R,define a bounded real continuous function u having the following properties: ( I ) for each integer n 2 0, x + u(x, n) takes the value 1 for x = rXn,is equal to 0 for x 4 I,, and 0 < ~ ( x 1,1 ) < 1 for all x E I ; ( 2 ) for each x E I, the function y+u(x,y) has the form n y /3 in each of the intervals ]-m,O[ and [n,n 11 (n E N). Show that there is no finite system of functions v i E rR(l), w, E KR(R)

+

+

(I

< i < n) such

I

that u(x, y )

-z ,l,

ui(x)w,(y)

< 1/4 in I x

R. (Suppose the contrary;

consider the functions 14.: x -+ ~ ( xI ?, ) in rRR(l), and observe that IIN. I/ = 1, llu. - u, II = 1 for rn # n. If there existed a finite dimensional subspace E of %,,(I) such that d(u., E) C 1/4 for each n, there would exist in E an infinite sequence (h,) such that Ilh,II = 2 and Ilh. - h,ll 2 1/2 for m # n , contradicting (5.10.1).) 3. Let E be the interval [0, 11 in R. (a) Show that if ak (1 < k < n) are n distinct points of E, the functions x + J x- a,\ are linearly independent in ZR(E). (b) Deduce from (a) that the function (x. y ) -> ( x - yI in E x E cannot be written as afinife sum

vi(x)wi(y),

where v i and w, are continuous in E.

i=1

4. Show that the Banach space %g(R)is not separable. (Use a similar method as the one applied in the Problem of Section 5.10.)

5. EQUICONTINUOUS SETS

Let H be a subset of the space 9YF(E) (E metric space, F normed space); we say that H is equicontinuous at a poinr xo E E if, for any E > 0, there is a 6 > 0 such that the relation d(xo,x ) 6 6 implies I/f ( x ) -f(xo)ll 6 E for every f E H (the important thing here being that 6 is independent o f f ) . We say that H is equiconrinuous if it is equicontinuous at every point of E.

Examples

(7.5.1) Suppose there exist two constants c, a > 0 such that llf(x) -f(y)II d c * (d(x, y))" for a n y f E H, and any pair of points x, y of E; then H is equicontinuous. (7.5.2) Any finite set of functions which are continuous at a point xo (resp. in E) is equicontinuous at xo (resp. equicontinuous). More generally any finite union of sets of functions which are equicontinuous at x o (resp. equicontinuous) is equicontinuous at xo (resp. equicontinuous).

142

VII SPACES OF CONTINUOUS FUNCTIONS

(7.5.3) Let (f,) be a sequence of functions in aF(E) which conoerges simply to a function g and is equicontinuous at x o (resp. equicontinuous). Then g is continuous at xo (resp. continuous).

Indeed, suppose Ilf,(xo) -f,(x)II < E for any x such that d(x, xo) < 6 and any n ; then, by the principle of extension of inequalities, we have IJg(x)- g(xo)I)Q E for any x such that d(x, xo) Q 6 . Q.E.D. (7.5.4) In the space @F(E), the closure of any equicontinuous subset is equicontinuous.

This follows at once from (3.13.13) and from the proof of (7.5.3).

(7.5.5) Suppose F is a Banach space, ( f , ) an equicontinuous sequence in %?F(E),and that for any point x of a dense subset D of E, the sequence (S,(s)) is convergent in F. Then the sequence (S,) converges simply to a (continuous) limit g. As F is complete, we have to prove that for each X E E, (S,(x)) is a Cauchy sequence in F. Now for any E > 0, there is a 6 > 0 such that the relation d(x, y ) Q 6 implies I)f,(x) --f,(y)Il Q 4 3 for every n. On the other hand, there exists y E D such that d(x, y ) < 6 , and by assumption, there is an no such that Ilf,(y) -f,(y)II < 4 3 for m > n o , n 2 n o . It follows that for m 2 no, n 2 n o , Ilf,(x> --S,(x)II Q E . Q.E.D. (7.5.6) Suppose E is a compact metric space, (f,)an equicontinuous sequence in WF(E). i’j”(S,) converges simply to g in E, it converges uniformly to g in E.

Given E > 0, for each x E E there is a neighborhood V ( x ) such that the relation y E V ( x ) implies Ilf,(x) -f,(y)ll Q ~ / 3for every n. Cover E by a finite number of neighborhoods V ( x , ) ; there exists an no such that for n 2 n o , we have llg(xi)-f.(xi)II < ~ / 3for all the indices i. But for any x E E, x belongs to one of the V ( x , ) ,hence we have I(f,(x) -f,(x,)\I < ~ / for 3 all n, and letting n tend to + CQ, this yields IIg(x) - g(x,)I(Q 43. Hence we have llg(x) -f,(x)II 6 E for any n > no and every x E E. Q.E.D.

(7.5.7) (Ascoli’s theorem) Suppose F is a Banach space and E a compact metric space. In order that a subset H of the Banach space %F(E) be relatively

5 EQUICONTINUOUS SETS

143

compact, necessary and suflcient conditions are that H be eguicontinuous and that, for each x E E, the set H(x) of all f ( x ) such that f~ H be relatiuely compact in F. (a) Necessity. If H is relatively compact for every c > 0, there exists a finite number of functionsf, E H such that for everyfE H, there is an index i such that I l f - f i l \ < 4 3 (3.17.5). From this it follows first that for every x E E we have /If ( x ) -fi(x)Ii < 4 3 , and as F is complete, this shows by (3.17.5) that H(x) is relatively compact. On the other hand, let V be a neighborhood of x such that y E V implies llfi(y) -fi(x)ll < 4 3 for every index i; then, for any f E H, y E V implies /lf(y) -f(x)il < E , which proves H is equicontinuous. (b) Syficiency. As WF(E) is complete by (7.1.3) and (7.2.1), we need only prove H is precompact (3.17.5). Given any E > 0, for each x E E, let V(.u) be a neighborhood of x such that y E V(x) implies I/f ( y ) -f(x)ll < 4 4 for every f E H. Cover E with a finite number of neighborhoods V(xi) (1 < i < m). On the other hand each of the sets H(xi) is relatively compact in F by assumption; so is therefore their union K; let ( c ~ be) a finite ~ ~ subset of K such that every point of K is in a ball of center one of the c j and radius 44. Let @ now be the (finite) set of all mappings i -+ q(i)of [l, m] into [ I , n] (intervals in N); for each cp E @, denote by L, the set of all functions f~ H such that, for every index i in [ I , m ] , we have Il.f(xi) - cq(illl < 44. Some of the L, may be empty, but from the definition of the c j it follows that H is covered by the union of the L,. To end the proof we need only show that the diameter of each L, is < E . Now i f f , g are both in L,, for each y E E there is an i such that y E V(xi), hence Ilf(y) -f(xi)ll < 4 4 and 11g(y) - g(xi)ll < 4 4 ; as Ij f ( x i )- g(xi)II < 4 2 by definition, we have lif(y) - g(y)Ij Q E for every y E E, i.e. ] I f 911 Q E . Q.E.D.

PROBLEMS

1. Let E be a metric space, F a normed space, H a bounded subset of %F(E). For each x E E, let 2 be the mapping u +-i i ( s ) of H into F, which is continuous and bounded. Show that in order that H be equicontinuous at x o , it is necessary and sufficient that be continuous at x o . the mapping x + 2 of E into V:(H) 2. Let E be a metric space, F a normed space, (,h)an equicontinuous sequence in %F(E). Show that the set of points x E E, such that (fi(x)) is a Cauchy sequence in F, is closed in E. 3. Let E be the interval [0, a[in R, and for any I ? , let f.(t)

= sin((t

+ 4nzn2)”2)

~

~

~

144

VII SPACES OF CONTINUOUS FUNCTIONS

in E. Show that the sequence (fn) is equicontinuous in E and converges simply to 0 in E, but that it is not relatively compact in the space %g(E)(show that if it were, it would converge unifi)rni/y to 0). 4. Let E be a metric space, F a normed space, (fJ a sequence of functions in %:(El, which is equicontinuous at a point N t E. Show that if the sequence (fn(a)) is convergent to b E F, then for any sequence (x.) in E such that lim x, = a, the sequence "+ m

5.

6.

7. 8. 9.

10.

11.

(fA(x,,))converges to b in F. Let E be a metric space, F a normed space. We say that a subset H of %,"(El is w i i f i r m l y equicourinuous if for any E > 0, there is a 6 > 0 such that the relation d(x, y ) < 6 implies llf(x) - f ( y ) l l < E for every,fe H. Any functionfe H is unifoimly continuous; conversely a finite set of uniformly continuous functions is uniformly equicontinuous. Show that for a bounded subset H of %F(E),the following properties are equivalent: (a) H is uniformly equicontinuous. (b) The mapping x + i of E into K,"(H) (Problem I) is uniformly continuous. (c) The mapping (u, x) + ~ ( xof) H x E into F (H being considered as a subspace of %F(E)) is uniformly continuous. Let E be a metric space, F a normed space, H a uniformly equicontinuous subset of %F(E)(Problem 5 ) ; show that the closure of H in %F(E) is uniformly equicontinuous. Let E be a compact metric space, F a normed space. Show that any equicontinuous subset of KF(E) is uniformly equicontinuous. Let E be a compact metric space, F a Banach space. Show that if a subset H of %,.(E) is relatively compact, the union of all the sets H(x), where x E E, is relatively compact in F (use Problem 5 of Section 7.2). Show that the conclusion of Ascoli's theorem (7.5.7) is still valid if instead of supposing H(x) relatively compact in F for every x E E, one only supposes H(x) relatively compact for all x E D, where D is a dense subset of E. Let E be a metric space, H an equicontinuous subset of'%Z(E). Show that the set A of points x E E such that H(x) is bounded in R is both open and closed in E. If E is compact and connected and if for one point x o E E, H(xo) is bounded in R, then H is relatively compact in ER(E). Let E be a metric space, H an equicontinuous subset of Xg(E). For each x E E, let u(x) = supf(x), w(x) = inff(x); show that if u (resp. w ) is finite at one point xo,it is IEH

(El1

+

finite and continuous in a neighborhood of x,; if u(xo) = u3 (resp. w(xo) = - .cl) then u(x) = (resp. w ( x ) = - m) in a neighborhood of xo . Conclude that the set of points x E E for which u ( x ) (resp. w(x)) is finite is both open and closed in E. 12. Let I = [a,61 a compact interval in R.A functionff %,,(I) is said to be lipschitzian for the constant k > 0 if, for every pair (x, x') of points of I, [ f ( x ) -f(x')l < k /x' - xi. Let K be the subset of 'C;n(l) consisting of all functionsfwhich are lipschitzian for k , and such that f ( a ) = 0. Show that the F-entropy H,(K) and the &-capacity C,(K) (Section 3.16, Problem 4) are given by the formulas

+

if

k(b - a )

is not an integer E

( [ t ] is the largest integer < t ) . (One may assume k = 1 by a suitable linear transformation. Suppose b - a = m, where n is an integer. Consider the set M,_ of the 2"-' functions g E K which are equal to affine functions in each of the intervals

6

REGULATED FUNCTIONS

145

and have in each such interval a derivative equal to 1 or to - 1 ; prove that the distance of any two distinct elements of M,-l is > 2/(n - I). Similarly, consider the subset Mi of M, consisting of the 2"-' functions of M, which are equal to x - a in the interval [a, a ( h - a)/n],and for each function g E MA, consider the set of all functions f E K such that g(x) - ( 2 / n ) < f ( x ) 6 g ( x ) for every x E I. Use a similar construction when (b - a)/&is not an integer.)

+

6. REGULATED FUNCTIONS

Let 1 be an interval in R, of origin a and extremity b (a or b or both may be infinite), F a Banach space. We say that a mapping f of I into F is a step-function if there is an increasing finite sequence of points of i (closure of 1 in R) such that xo = a, xn = b, and that f is constant in each of the open intervals ] x i ,x i + l [(0 6 i < n - 1). For any mapping f of I into F and any point x E I distinct from b, we lim f ( y ) exists; we then write the say that f has a limit on the right if Y€l,Y>X Y-x

limit f ( x + ) . Similarly we define for each point x E I distinct from a, the limit on the feft off at x, which we write f ( x - ) ; we also say these limits are one-sided limits off. A mapping f of I into F is called a regulated function if it has one-sided limits at every point of I . It is clear that any step-function is regulated.

(7.6.1) In order that a mapping f of a compact interid I = [a, b ] into F he regulated, a necessary and suficient condirion is thatf be the limit of a unformly convergent sequence of step-functions. (a) Necessity. For every integer n, and every x E I, there is an open interval V ( x ) = ] y( x) ,z(x)[ containing x, such that Ilf(s) -f(t)II < I/n if either both s, t are in ]y(x),x[ n I or both in ]x, z ( x ) [n I . Cover I with a finite number of intervals V ( x , ) ,and let ( c ~be the ) strictly ~ ~ increasing ~ ~ sequence consisting of the points a, b, x i , y ( x i ) and z(xi). As each c j is insome V(xi), c j + ]is either in the same V ( x i )or we have c j + , = z(xi), f o r j d m - 1 ; in other words if s, t are both in the same interval ] c j ,c ~ + ~ then [ , IIf(s) - f(t)ll < I/n. Now define g, as the step-function equal to f at the points c j , and at the midpoint of each interval ] c j , c j + ] [ , and constant in each of these intervals. It is clear that Ilf- grill d l/n.

~

VII SPACES OF CONTINUOUS FUNCTIONS

146

(b) Suficiency. Suppose f is the uniform limit of a sequence (f,) of step-functions. For each E > 0 there is an n such that l\f-fnll Q 43; now for each x E I , there is an interval ]c, d[ containing x and such that Ilf,(s) -f,(t)ll < ~ / 3if both s and t are in ]c, x [ or both in ]x, d [ ; hence under the same assumption we have IIf(s) -f(t)II Q E , and this proves the existence of one-sided limits off at x, since F is complete (3.14.6). Another way of formulating (7.6.1) is to say that the set of regulated functions is closed in BF(E), and that the set of step-functions is dense in the set of regulated functions. (7.6.2) Any continuous mapping of an interval I c R into a Banach space is regulated; so is any monotone mapping of I into R.

This follows from the definition, taking into account (3.16.5) and (4.2.1).

PROBLEMS

1. Let f be a regulated mapping of an interval I c R into a Banach space F. Show that for each compact subset H of I, f ( H ) is relatively compact in F; give an example showing that f(H) need not be closed in F. 2. The function f(x) = x sin(l/x) ( f ( 0 )= 0) is continuous, hence regulated in I = [0, I], and the function g(x) = sgn x (g(x) = 1 if x > 0, g(0) = 0,g(x) = - 1 if x > 0) is regulated in R, but the composed function g o f is not regulated in I. 3. Let I = [a, b ] be a compact interval in R. A function of bounded variation in I is a mapping fof I into a Banach space F, having the following property: there is a number V >, 0 such that,for m y strictly increasing finite sequence (fi)ogrgnof pointsof I, the inequality n- 1

C llf(ti+~)-f(ti)ll

i=o

G V holds.

(a) Show that f(1) is relatively compact in F (prove that f(1) is precompact, by an indirect proof). (b) Show thatfis a regulated function in I (use (a) and (3.16.4)). (c) The function g defined in [0, I ] as equal to xz sin(l/xz) for x # 0 and to 0 for x = 0 is not of bounded variation, although it has a derivative at each point of I.

CHAPTER Vlll

DIFFERENTIAL CALCULUS

The subject matter of this chapter is nothing else but the elementary theorems of calculus, which however are presented in a way which will probably be new to most students. That presentation, which throughout adheres strictly to our general “geometric” outlook on analysis, aims at keeping as close as possible to the fundamental idea of calculus, namely the “ local ” approximation of functions by linear functions. In the classical teaching of calculus, this idea is immediately obscured by the accidental fact that, on a one-dimensional vector space, there is a one-to-one correspondence between linear forms and numbers, and therefore the derivative at a point is defined as a number instead of a linear form. This slavish subservience to the shibboleth of numerical interpretation at any cost becomes much worse when dealing with functions of several variables : one thus arrives, for instance, at the classical formula (8.9.2) giving the partial derivatives of a composite function, which has lost any trace of intuitive meaning, whereas the natural statement of the theorem is of course that the (total) derivative of a composite function is the composite of their derivatives (8.2.1), a very sensible formulation when one thinks in terms of linear approximations. This “ intrinsic” formulation of calculus, due to its greater “ abstraction,” and in particular to the fact that again and again, one has to leave the initial spaces and to climb higher and higher to new “function spaces” (especially when dealing with the theory of higher derivatives), certainly requires some mental effort, contrasting with the comfortable routine of the classical formulas. But we believe that the result is well worth the labor, as it will prepare the student to the still more general idea of calculus on a differentiable manifold, which we shall develop in Chapters XVI to XX. Of course, he will observe that in these applications, all the vector spaces which intervene have 147

148

Vlll

DIFFERENTIAL CALCULUS

finite dimension; if that gives him an additional feeling of security, he may of course add that assumption to all the theorems of this chapter. But he will inevitably realize that this does not make the proofs shorter or simpler by a single line; in other words, the hypothesis of finite dimension is entirely irrelevant to the material developed below ; we have therefore thought it best to dispense with it altogether, although the applications of calculus which deal with the finite dimensional case still by far exceed the others in number and in importance. After the formal rules of calculus have been derived (Sections 8.1 t o 8.4), the other sections of the chapter are various applications of what is probably the most useful theorem in analysis, the mean value theorem, proved in Section 8.5. The reader will observe that the formulation of that theorem, which is of course given for vector valued functions, differs in appearance from the classical mean value theorem (for real valued functions), which one usually writes as an equality J’(h) - f ( a ) =f’(c)(b - a). The trouble with that classical formulation is that: ( I ) there is nothing similar to it as soon as f has vector values or when there are a finite number of points where f ’ is not defined; (2) it completely conceals the fact that nothing is known on the number c, except that it lies between a and b, and for most purposes, all one need know is that f ’ ( c ) is a number which lies between the g.1.b. and 1.u.b. off’ in the interval [a, b] (and not the fact that it actually is a value off’). In other words, the real nature of the mean value theorem is exhibited by writing it as a n inequality, and not as an equality. Finally, the reader will probably observe the conspicuous absence of a time-honored topic in calculus courses, the “ Riemann integral.” It may well be suspected that, had it not been for its prestigious name, this would have been dropped long ago, for (with due reverence t o Riemann’s genius) it is certainly quite clear to any working mathematician that nowadays such a “ theory” has at best the importance of a mildly interesting exercise in the general theory of measure and integration (see Section 13.9, Problem 7). Only the stubborn conservatism of academic tradition could freeze it into a regular part of the curriculum, long after it had outlived its historical importance. Of course, it is perfectly feasible to limit the integration process to a category of functions which is large enough for all purposes of elementary analysis (at the level of this first volume), but close enough to the continuous functions t o dispense with any consideration drawn from measure theory; this is what we have done by defining only the integral of regulated functions (sometimes called the “ Cauchy integral ”). When one needs a more powerful tool, there is no point in stopping halfway, and the general theory of (“ Lebesgue”) integration (Chapter XIII) is the only sensible answer.

1 DERIVATIVE OF A CONTINUOUS MAPPING

149

1. DERIVATIVE OF A C O N T I N U O U S MAPPING

Let E, F be Banach spaces (both real or both complex) and A a n open subset of E. Let h g be two continuous mappings of A into F; we say that f and g are tangent at a point x, E A if lim lif(x) - g(x)l)/llx- xoII = 0 ; x-xn,x+xo

this implies of course that f ( x o )= g(xo). We note that this definition only depends on the topologies of E and F; for i f h g are tangent for the given norms on E and F, they are still tangent for equivalent norms (Section 5.6). Jff, g are tangent at xo, and g, h tangent at xo, thenf, h are tangent at x, as follows from the inequality 11 f(x) - h(x)(j < lif(x) - g(x)ll + IIg(x) - h(x)/J. Among all functions tangent at x, to a function f , there is at most one mapping of the form x - f ( x , ) + u(x - x,) where u is linear. For if two such functions x +f(xo) + u l ( x - x,), x 4f ( x o )+ uz(x - x,) are tangent at xo, this means, for the linear mapping u = u1 - u 2 ,that lim Ilu(y)ll/llyll = 0. But this implies L' = 0, for if, given E > 0, there is r > 0 such that ljyll < r implies llv(y)ll d E llyll, then this last inequality is still valid for any x # 0, by applying it to y = rx/ilxil; as E is arbitrary, we see that u(x) = 0 for any x. y-+O,yfO

We say that a continuous mapping f of A into F is diflerentiable at the point x, E A if there is a linear mapping u of E into F such that x + f ( x o ) + u(x - x,) is tangent to f at x,. We have just seen that this mapping is then unique; it is called the derivative (or total derivative) o f f at the point x,, and writtenf'(x,) or Df(x,).

(8.1 .I)If the continuous mapping f of A into F is diferentiable at the point xo, the deriuative f '(x,) is a continuous linear mapping of E into F.

Let u = f ' ( x o ) .Given E > 0, there is r such that 0 < r < 1 and that lltll < r implies llf(xo t ) -f(xo)ll < 4 2 and llf(xo + t ) -f(xo) - 4l)ll < & IllIlP; hence IltlI d r implies Ilu(t)ll < E , which proves u is continuous by (5.5.1).

+

The derivative (when it exists) of a continuous mappingfof A into F, at a point x, E A, is thus an element of the Banach space Y(E; F) (see Section 5.7) and not of F. In what follows, for u E 9 ( E ; F) and t E E, we will write u . t instead of u ( t ) ; we recall (Section 5.7) that Ilu .tll < llull . lltll and that

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When E has finite dimension n and F has finite dimension m , f ' ( x o ) can thus be identified to a matrix with nz rows and n columns; this matrix will be determined in Section 8.10. Examples

(8.1.2) A constant function is differentiable at every point of A, and its derivative is the element 0 of 9 ( E ; F). (8.1.3) The derivative of a continuous h e a r mapping u of E into F exists at every point x E E and Du(x) = u. For by definition u(xo) + u(x - x o ) = u ( x ) . (8.1.4) Let E, F, G be three Banach spaces, ( x , y ) --* [x . y ] a continuous bilinear mapping of E x F into G. Then that mapping is differentiable at every point (x,y ) E E x F and the derivative is the linear mapping (s, t ) --* [x . tl [s . y l .

+

For we have [(x

+ s) . ( y + t ) ] - [ x . y ] - [ x . t ] - [ s . y ] = [s. t ]

and by assumption, there is a constant c > 0 such that l\[s * t]ll < c . llsll * (It/l (5.5.1). For any E > 0, the relation sup(llsll, Iltll) = II(s, t)l\ Q E / C implies therefore

"I

+ ). . ( y + t>l - [x ul - [x . 11 - 1s - YlII *

,< & IKS, t)11

which proves our assertion. That result is easily generalized to a continuous multilinear mapping. (8.1.5) Suppose F = F, x F, x ... x F, is a product of Banach spaces, and f = (f i , . . . ,f,) a continuous mapping of an open subset A of E into F. In order that f be differentiable at xo , a necessary and suficient condition is that each fi be differentiable at xo , and then f ' ( x o ) = ( f ; ( x o ) ,. . . ,fA(xo)) (when 2 ( E ; F) is identiJied with the product of the spaces Y(E; FJ).

Indeed, any linear mapping u of E into F can be written in a unique way u = (ul, . . . , u,), where u i is a linear mapping of E into F i , and we have by definition \lu(x)II = sup(llul(x)(~, . . . , ~ ~ u , ( x ) ~whence ~), it follows

2 FORMAL RULES OF DERIVATION 151

(by (5.7.1) and (2.3.7)) that llull

= sup(jlu, I/,

identification of 2 ( E ; F) with the product

m

. . . , IIuJ), which allows the

fl Y(E; Fi). From

the defini-

i= I

tion, it follows at once that u is the derivative off at x , if and only if ui is the derivative of f i at x, for 1 < i Q m. Remark. Let E, F be complex Banach spaces, and E,, F, the underlying real Banach spaces. Then if a mapping f of an open subset A of E into F is differentiable at a point x o , it is also differentiable with the same derivative, when considered as a mapping of A it7tO F, (a linear mapping of E into F being also linear as a mapping of Eo into F,). But the converse is not true, as the example of the mapping z + Z (complex conjugate) of C into itself shows at once; as a mapping of R2 into itself, u : z + 5 (which can be written (x,y ) -+ ( x , - y ) ) is differentiable and has at each point a derivative equal to u, by (8.1.3); but u is not a complex linear mapping, hence the result. We return to that question in Chapter IX (9.10.1). When the mapping f of A into F is differentiable at every point of A, we say that f is differentiable in A ; the mapping x + f ’ ( x ) = Df(x) of A into Y(E; F) will be writtenf’ or Dfand called the derivative o f f in A.

2. FORMAL RULES OF D E R I V A T I O N

(8.2.1) Let E, F, G be three Banach spaces, A an open neighborhoodof x, E E, f a continuous mapping of A into F, yo = f ( x , ) , B an open neighborhood of y o in F, g a continuous mapping of B into G. Then iff is differentiable at x, and g differentiable at y o , the mapping h = g o f (which is defined and continuous in a neighborhood of x,) is differentiable at x, , and we have h’(x0) = S’(Y0) o f ‘(xo).

By assumption, given E such that 0 < E < 1, there is an r > 0 such that, for 1)s11 < r and litil Q r, we can write f ( x 0 + s> =f(x,>+f’(xo) . s + 01(s) S(Y0 + t ) = d Y 0 ) + S’(Y0) . t

+ OAt)

with i\ol(s)I\ Q ~ l l s l land l\02(t)II< ~jltll.On the other hand, by (8.1.1) and (5.5.1), there are constants a, b such that, for any s and t, llf’(xo>*

SII

Q a llsll

and

IlS‘(Y0)

. tll

< b lltll

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DIFFERENTIAL CALCULUS

hence Ilf’(x0)

*

+ o,(s)ll G (a + 1) llsll

for /Is/l < r. Therefore, for ~~s~~ < r/(a and

Iloz(f’(x0)

+ I),

. s + Ol(S))lI

II9’(Yo)

we have Q (a f

I)&Ibll

. Ol(S>ll G bE llsll

hence we can write

h(x0 + $1 = d Y 0 +f’(xo) * s + OI(S)) = S(Y0) + S’(Y0) . (f’(xo>. s) + ads) with Ilo3(s)ll Q (a

+ b + 1)E I l ~ l l ,

which proves the theorem.

(8.2.1)has of course innumerable applications, of which we mention only the following one: (8.2.2) L e t x g be twto continuous mappings of the open subset A of E into F. I f f and g are differentiable at x, , so are f + g and af ( a scalar), and we have (f+ g)‘(xo)= f ‘ ( X 0 ) + g’(x0) and (af)’(x,>= af‘(x0). The mapping f + g is composed of (u, v) 4u + v, mapping of F x F into F, and of x + ( f ( x ) , g ( x ) ) , mapping of A into F x F; both are differentiable by (8.1.3)and (8.1.5),and the result follows (for f + g ) from (8.2.1).For af the argument is still simpler, using the fact that the mapping u+au of F into itself is differentiable by (8.1.3).Of course, (8.2.2) could also be proved very simply by direct arguments. Let E, F be two Banach spaces, A an open subset of E, B a n open subset of F. If A and B are homeomorphic, and there exists a differentiable homeomorphism f of A onto B, it does iiot follow that, for each x, E A, f ‘ ( x o ) is a linear homeomorphism of E onto F (consider e.g. the mapping 5 + of R onto itself).

c3

(8.2.3) Let f be a homeomorphism of an open subset A of a Banach space E onto an open subset B of a Banach space F, g the inverse homeomorphism. Suppose f is differentiable at the point x, ,and f ’(x,) is a linear homeomorphism of E onto F; then g is differentiable at y o = f(xo) and g‘(y,) is the inverse mapping t o f ‘ ( x o )(cf. (10.2.5)).

2 FORMAL RULES OF DERIVATION 153

+

By assumption, the mapping s + f ( x , s) - f ( x o ) is a homeomorphism of a neighborhood V of 0 in E onto a neighborhood W of 0 in F, and the inverse homeomorphism if t +g(yo + t ) -g(y,). By assumption, the linear mappingf’(xo) of E onto F has an inverse u which is continuous, hence (5.5.1) there is c > 0 such that Iju(t)/I 9 c Iltll for any t E F. Given any E such that 0 < E < 1/2c, there is an r > 0 such that, if we write f ( x o + S ) - f ( x o ) = f ’ ( x , ) a s + ol(s), therelation llsll < r implies Ilol(s)II 9 E IIsII. Let r‘ now be a number such that the ball lltli 9 r’ is contained in W and that its image by the mapping t -+ g(y, t ) - g(y,) is contained in the ball llsll 9 r. Let z = g(y, + t ) - g(y,); by definition, for lltll < r ’ , this equation implies t = f ( x , z ) - f ( x o ) and as llzll 9 r, we can write t = f ’ ( x , ) . z + ol(z), with liol(z)II 9 E llzll. From that relation we deduce

+

+

u * t = u * (f’(x,) * z)

+u

*

ol(z) = z

+u

*

ol(z)

+

by definition of u, and moreover IIu * ol(z)II 9 c llol(z)IJ < CE llzll < 1 1 ~ 1 1 , hence IIu . t/l 2 llzll - 4 llzll = -t llzll; therefore llzll < 2Ilu. tll < 2cIltl1, and finally IIu . ol(z)II < CE llzll < 2c% Iltll. We have therefore proved that the relation lltll < r‘ implies IIg(y, + t ) - g(y,) - u . t 11 < 2cZc Iltll, and as E is arbitrary, this completes the proof. The result (8.2.3)can also be written (under the same assumptions)

PROBLEMS 1. Let E be a real prehilbert space. Show that in E the mapping x + llxli of E into R is differentiable at every point x # 0 and that its derivative at such a point is the linear mapping s+(sIx)/IIx/l. 2. (a) In the space (co) of Banach (Section 5.3, Problem 5) show that the norm x + J/xlj is differentiable at a point x = (8.) if and only if there is an index no such that > I f . 1 for every n # n o . Compute the derivative. (b) In the space I’ of Banach (Section 5.7, Problem I ) , show that the norm x + llxll is not differentiable at any point (use (8.1.1) and Problem l(c) of Section 5.7).

ltnol

3. Let f be a differentiable real valued function defined in an open subset A of a Banach space E. (a) Show that if at a point xo E A,freaches a relative maximum (Section 3.9, Problem 6), then Df(xo) = 0. (b) Suppose E is finite dimensional, A is relatively compact,fis defined and continuous in A, and equal to 0 in the boundary of A. Show that there exists a point x,, E A where D f ( x o )= 0 (“Rolle’s theorem”; use (a) and (3.17.10)).

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3. DERIVATIVES IN SPACES

O F C O N T I N U O U S LINEAR F U N C T I O N S

(8.3.1) Let E, F , G be three Banach spaces. Then the mapping (u, v) -+ v u (also written vu) of 9 ( E ; F) x 9 ( F ; C ) into 9 ( E ; G) is diferentiable, and the derivative at the point (uo , vo) is the mapping (s, t ) -+ vo s + t uo . 0

0

0

If we observe that, by (5.7.5), the mapping (u, v) -+ v u is bilinear and continuous, the result is a special case of (8.1.4). 0

(8.3.2) Let E , F be two Banach spaces, such that there exists at least a linear homeomorphism of E onto F. Then the set 2 of these linear homeomorphisms is open in 9 ( E ; F ) ; the mapping u -+ u - l of iff onto the set Y? of linear homeomorphisms of F onto E is continuous and diferentiable, and the derivative of u -+ u-l at the point uo is the linear mapping (of Y ( E ; F ) into Y(F; E ) )

-'

s --f

-24;'

0

s

0

u;'.

1. We consider first the case F = E, and write 1, for the identity mapping of E. Then:

(8.3.2.1) If llwll < 1 in 9 ( E ; E), the Zinear mapping 1, + w is a homeomorphism, its inverse (1, + w)-' is equal to the sum of the absolutely convergent series

m

C (n=O

l)nw", and we have

(8.3.2.2)

11(IE

We have

N n=O

+ w)-'

- l E + wII

IIwII" = (1 - IIw IIN")/(l -

- Ilwll>.

~~K'~\2/(1

Ilwll)

< l/(l - Ilwll),

hence, by

m

(5.7.5), (5.3.1), (5.3.2) and (5.7.3), the series convergent in 9(E; E). Moreover, we have (1E+W)(1E-W+w2

= (1, - w

(-1)"

is absolutely

n=O

+"'+(-l)NWN)

+ w2 +

* "

+ ( - l ) N W N ) ( l E + w) = 1 E - ( - l ) N + ' W N + l .

and as w N + l tends to 0 with 1/N, we have by definition and by (5.7.5),for the W

element v

= n=O

( - 1 ) " ~ ~ of 9 ( E ; E), (1,

+ w)v = v ( l E + w) = l E , which

proves the first two statements; the inequality (8.3.2.2) follows from the relation (1, + w)-' - 1, + w = w2(lE - IY f M!2 and from (5.7.5) and (5.3.2).

+

. . a ) ,

4

DERIVATIVES OF FUNCTIONS OF ONE VARIABLE

155

2. Consider now the general case; suppose s ~ 9 ( EF); is such that ( ( s ( (. Ilui'll < I ; then the element 1, + u0-'s, which belongs to 9 ( E ; E), has an inverse, due to (5.7.5) and (8.3.2.1);as we can write uo s = uo(1, ui's), the same is true for uo + s, the inverse being (1, u;'s)-'u,'; hence we have

+ +

+

(ug

+ s)-'

Applying (8.3.2.2)to Il(u0

+ s)-'

- t!;' = ( ( 1 E

UJ = ui's,

- u;' Il(u0

- 1E)u;'.

we obtain, for llsll < l/llu;'II

+ u;'su;'ll

Therefore, if we take llsll

+ u,'s)-'

d Ilu,'113. lIsll'/(l - lu;'II

*

Ilsll).

< 1 / 2 ~ ~ u ~we' ~ have l,

+ s)-'

- u;'

+ u,'su,'ll

d c lls11'

' ~ this ~ ~ ends , the proof. with c = 2 ~ ~ u ; and

4. DERIVATIVES OF F U N C T I O N S O F O N E VARIABLE

When we specialize E to a one-dimensional vector space (identified to R or C), we know that 9 ( E ; F) is naturally identified to F itself, a vector b E F being identified to the linear mapping 5 -+ b( of E into F (5.7.6). If f is a differentiable mapping of an open set A c E into F, its derivative D f ( t 0 )at a point toE A is thus identified to a vector of F, and the mapping Df to a mapping of A into F. If F itself is one-dimensional (identified to R or C), we obtain the classical case of the derivative (at a point) as a number. The general results obtained above boil down in that last case to the classical formulas of calculus; for instance, (8.3.2),when E and F are one-dimensional, is simply the formula giving the derivative of 1/4 as equal to - 1/(' for 5 # 0. We explicitly formulate the following consequence of (8.2.1):

(8.4.1) Let E, F be two real (resp. complex) Banach spaces, f a differentiable mapping of an open subset A of E into F, g a diferentiable mapping of an open subset I of R (resp. C) into A; then the derivative at 5 E I of the composed mapping h =f .g of I into F is the vector of F equal to Df(g(<)).g'(c) (remember g'(5) is in E, and Df(g(())in 9 ( E ; F)). Remarks. Suppose F is a complex Banach space, f a differentiable mapping of an open subset A c C into F; its derivative at z E A is thus identified with a vector of F. Let now g be a differentiable mapping of an open subset I of R into C (considered as the underlying two-dimensional real vector space);

156

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then f g is a differentiable mapping of I into the underlying real Banach space F, of F, and (8.4.1) shows that its derivative at a point ( E I is g'({)(Df(g([))) (remember here g'(5) is a complex number). When E = R and F is a real Banach space, the notion of derivative can be greatly generalized: for any subset J c R and any point toE J such that 5, is a cluster point of J - {to},we can define, for a mappingfof J into F, the derivatioe off at 5, ( ~ V t respect h to J) as the limit (when it exists) liln

<+to, < E J-ITo)

(f(t>-f(to))/(5- 5 0 ) .

When the limit exists, we say that f is d e r e n t i a h l e at T o , ,r,itlz respect to J. We shall only consider the case in which J is an interval of R ; then at the interior points of I, the derivative with respect to J coincides (when it exists) with the usual one; at the origin a (resp. extremity fi) of J, when it belongs to J, the derivative offwith respect to J is also called the derivatiw on the right (resp. on the left) o f f at the point c1 (resp. p) and written f;(a) or D + f ( a ) (resp. f,'(P) or D-f(/l)). Theorem (8.4.1) is still valid when in the assumptions we suppose I is an interval and g has a derivative with respect to I at 5 ; then iffis differentiable in A , j og has a derivative at 5 with respect to I given by the same formula (g'(5) being replaced by the derivative of g with respect to I). The proof is that of (8.2.1) with the obvious modifications. We omit the most usual consequences of that theorem, such as the result corresponding to (8.2.2).

PROBLEMS

1. (a) Let f be a continuous mapping of an interval I c R into a Banach space E. In order that f be differentiable at an interior point xo of I , it is necessary and sufficient that ( f ( x o h) - f ( x o - k ) ) / ( h k ) have a limit in E when the point (h, k ) tends to (0,O) in the set of pairs such that h > 0, k > 0. (b) The real functionfequal to x z sin(l/x) for x # 0, to 0 for x 0, is differentiable in R, but ( f ( x ) - f ( y ) ) / ( x - y ) has no limit when ( x , y ) tends to ( 0 , O ) in the set of pairs such that x > 0, y > 0, x # y . (c) In the interval I = [O, I], the sequence of continuous functions f. is defined as follows: fo(t) = t ; for each n > 1, fn has the form at fl in each of the 3" intervals

+

+

:

+

k 3"

-Sf<-

moreover

k+' 3"

for O < k < 3 " - l ;

4

DERIVATIVES

OF FUNCTIONS OF ONE VARIABLE

157

Show that the sequence (f.) converges uniformly in I towards a continuous function which has no derivative at any point of I (use (a)). 2. Letfbe a continuous mapping of an open interval 1 c R into a Banach space E, which has at every point t E 1 both a derivative on the left f & t ) and a derivative on the right

f&).

(a) Let U be a nonempty open subset of E, A the set of points t E I such that&(/) E U. For any a > 0, let B. be the subset of I consisting of points t such that there is at least a point s E I for which t - c( < s < t and ( f ( t )- f ( ~ ) ) / ( t- s) E U ; show that B. is open and that A n CB. is denumerable (use Problem 3 of Section 3.9). Conclude from that result that the set of points t E A such thatf,'(t) 4 0 is at most denumerable. (b) Deduce from (a) that the set of points t t I such that & ( t ) # f i ( f ) is at most denumerable. (Observe first that f(1) is a denumerable union of compact metric spaces, and by considering the closed vector subspace of E generated by f(1). reduce the problem to the case in which the topology of E has a denumerable basis (U.) of open sets; then remark that for every pair of distinct points a, b of E there is a pair of sets U,, U, such that a~ U p , h t U, and U p n U,= 0.) 3. (a) Let f be defined in R2 by the conditions

Show that for any x

E

R 2 and any y E R2, the limit lim ( f ( x +ty) r-o.r#O

- f ( x ) ) / t=

g(x,y)

exists but that y +g(O,y) is not linear (hence f is not differentiable at the point 0). (b) Let f be defined in RZ by the conditions

Show that the limit g(x, y ) exists for every x and y and y +g(x, y) is linear for every E R2, but that f i s not differentiable at the point 0. (Consider the points (El, such that = (a) Let f be a continuous mapping of an open subset A of a Banach space E into a Banach space F. We say that at xo E A the functionfisqrrasi-differentiable if there exists a linear mapping f r of E into F, having the following property: for any continuous mappingg of I = [0,1] into A such that g(0) = xo and that the derivativeg'(0) of g at 0 (with respect to I) exists, then t -f(g(f)) has at the point t = 0 a derivative (with respect to I ) equal to rc(g'(0)).The linear mapping I I is then called a quasi-derivative o f f a t x o . Show that if f is quasi-differentiable at x o , its quasi-derivative is unique. Extend property (8.2.1) to quasi-differentiable mappings. (b) Show that i f f is quasi-differentiable at x o , its quasi-derivative u is a continuous linear mapping of E into F. (Suppose, as one may, that xo = O , f ( x o ) = 0. Use contradiction: if u is not bounded in the ball B(0; I), there exists a sequence (a,) of vectors in E such that llanll= I , and a sequence (t.) of numbers > 0, such that lim t, = 0 and

x

4.

t2)

t2 t:.)

+

n-m

that Ilt;'f(tnan)ll = a. tends to co; one can suppose that the sequences ( I , ) and 1,) are decreasing and tend to 0. Define a continuous mapping g of [0, 11 into E such that g(0) = 0, that g'(0) exists and is equal to 0, and that g ( J i t,) = faan).) 5. (a) Let E, F be two Banach spaces,fa continuous mapping of an open subset A of E into F. Show that if is diffcrentiable at x o t A, it is quasi-differentiable at xo and its quasi-derivative is equal to its derivative.

(din

158

Vlll DIFFERENTIAL CALCULUS

(b) Suppose E has finite dimension. Show that iff is quasi-differentiable at xo E A,

f is differentiable at xo . (Use contradiction: let u be the quasi-derivative o f f at xo , and suppose there is a > 0 and a sequence (x,) of points of A, tending to xo , such that llf(x,) -f(xo) - u (x. - xo)II> aIIx, - xo(I. Using the local compactness of E, show that one may suppose that the sequence (((x, - xo 11) is decreasing, and that the sequence of the vectors z. = (x,- xo)/Ilx,- xo 11 tends to a limit in E; then define a continuous mapping g of [0, 11 into E such that g(0) = xo , that g'(0) exists, but that

-

u(g'(0)) is not the derivative of t + f ( g ( t ) ) at t = 0.)

6. Let I = [0, I], and let E be the Banach space WR(I). In order that the mapping x -+ llxll of E into R be quasi-differentiable at a point xo , it is necessary and sufficient that the function 1 Ixo(t)l reaches its maximum in I at a single point to E I ; the quasi-derivative of x + l/xll at xo is then the linear mapping u such that u(z) = z(to) if -+

xo(to)> 0, u(z) = -z(to)

if xo(to)< 0 (compare Section 8.2, Problem 3). (To prove the condition is necessary, suppose lxol reaches its maximum at two distinct points t o , tl at least; let y be a continuous mapping of I into itself, equal to 1 at t o , to 0 at t l ; examine the behavior of ( I/xo hyll - /IxoIl)/has the real number h # 0 tends to 0. To prove the condition is sufficient,let h + z, be a continuous mapping of I into E, having a derivative a f E at h = 0 and such that zo =O; observe that if r, is the largest number in I (or the smallest number in I) where t Ixo(t) z,(t)l reaches its maximum, then t, tends to to when h tends to 0.) Deduce from that result that the mapping x -+ j/x/I of E into R is not differentiable at any point (compare to Section 8.2, Problem 2). 7. Letfbe a continuous mapping of an open subset A of a Banach space E into a Banach space F. Supposefis lipschitziun in A : this means (7.6, Problem 12) that there exists a constant k > 0 such that llf(xl) -f(xZ)ll < k I/xl - xz/I for any pair of points of A. Let xo E A, and suppose there is a linear mapping u of E into F such that, for any vector a # 0 in E, the limit of (f(xo f a t ) --.f(x0))/t when t # 0 tends to 0 in R,exists and is equal to u(a). Show that f i s quasi-differentiable at xo . llu rb /I of 8. (a) Let u , b be two points in a Banach space E. Show that the mapping t R into itself has a derivative on the right and a derivative on the left for every t E R (prove that if 0 < t < s, then ([la bril- Ilall)/t < (ila bsll- Ilall)/s and use (4.2.1)). (b) Let u be a continuous mapping of an interval I E R into E. Show that if at a point t o E I, u has a derivative on the right, then t Ilu(t)ll has at to a derivative on the right and

+

+

-+

-+

+

+

+

--f

(D+ IlulI)(to)

< IID+u(to)ll

(apply (a)). (c) Let U be a continuous mapping of I into Y(E; E). Show that if at a point to E I, U has a derivative on the right and U(t0) is a linear homeomorphism of E onto itself, then the mapping f l\(U(t))-lII= f ( t ) , which is defined in a neighborhood of t o , has a derivative on the right at t o , and that

-

5. T H E M E A N VALUE THEOREM

Let I = [ct, p] be a compact interval in R, f a continuous mapping of I into a Banach space F, cp a continuous mapping of I into R. We suppose that there is a denumerable subset D such that, for each 5 E I - D, f and cp have both a derivative at 5 with respect to I (8.4), and that (1 f'(5)II < cp'(5j. Then I l f (PI - f(4I1 G cp(P) - c p ( 4

(8.5.1)

5 THE MEAN VALUE THEOREM

159

Let n -+ p n be a bijection of N onto D ; for any E > 0, we will prove that left hand side being independent of E , this will complete the proof. Define A as the subset of I consisting of the points 5 such that, for a < 5 < 5 ,

IIf(p) -f(a)II < cp(p) - cp(a) + E(P - a + 2 ) ; the

It is clear that a E A ; if 5 E A and a < v] < 4, then v] E A also, by definition; this shows that if y is the 1.u.b. of A, then A must be either the interval [a, y [ or the interval [a, y ] ; but in fact, from the definition of A it follows at once that A = [ E , y ] . Moreover, from the continuity of f a n d cp it follows that (8.5.1.1)

Ilf(Y) -f(.>II< d Y ) - cp(4 + 4 Y - 4 + E

c

2-"

Pn
and therefore we need only prove that y = p. Suppose y < p ; if y $ D, then from the definition of the derivative, it follows that there is an interval [y, y A] contained in I such that, for y < 5 < y + A

+

Ilf(5) -f(r) -f'(r>(5 - Y)ll < (&/2)(1- Y) and hence

and from (8.5.1 .l) we deduce

contrary to the definition of y. If y E D, let y = p m ; it follows from the continuity offand cp that there is an interval [y, y + A] contained in I, such that for y < 5 c y + A ,

Ilf(5) -f(Y)ll

< (&/2)2-"7

I d 5 ) - cp(Y)l

hence, from (8.5.1 .I) we deduce again

IIf(5) -f(.)II

< rp(5) - d")+ 4 Y - 4 + E < Cp(5)

- Cp(a) + E(( - a) 4-

E

< (E/2)2-"

c 2-" c 2-"

Pn
Pn
and we reach again a contradiction. Q.E.D. The most important case is that in which q(5) = M(< - ct) with M > 0:

DIFFERENTIAL CALCULUS

160

Vlll

(8.5.2)

If there is a denunzerable subset D of I such that, f o r each ( E I - D, I)(’fl < M, then -f(.)Il G M(B - 4.

f has at 5 a derivative bvith respect to I such that

llf(P)

For real-valued functions, the same argument as in (8.5.1) proves the first part of: (8.5.3) Sippose cp is a continuous mapping of I into R such that, at every point 5 E I - D, cp has a derivative usitli respect to I, and m d < M. Then rn(P - a ) < &I) cp(a) - < M(B - a ) ; and in fact (PI(()

n1(B - a) < d

P ) - 44). < M(P - a),

except when cp(t)= cp(a) + m(t - a ) or cp(()

= cp(a)

+ M(( - a ) f o r 5 E I.

To prove the second part, observe that by the first part, the function is not identically 0, then for the other inequality.

(~(5)- cp(a) - m(t - a ) is increasirig in I; if it cp(0) - cp(a) - m(P - a) > 0. Similar argument

In a normed space E, we define the segment joining two points a, b as the set of points a + t ( b - a ) with 0 < 5 < 1.

Let E, F be two Banach spaces, f a coritinuous mapping into F of a neighborhood of a segment S joining tii’o points xo, xo + t of E. I f f is differentiable at every point of S , then (8.5.4)

Ilf(x0

+ 1 ) -f(xo)ll

6 lltll . SUP llf’(x0 045s 1

+ tt>ll.

Consider the mapping g of the interval I = [0, I ] into F defined by g ( ( ) = f ( x o + t t ) ; by (8.4.1), (8.2.2) and (8.1.3), g is differentiable at every point of I (with respect to I) and its derivative is f ’ ( x o + ( t ) . t ; hence the result by (8.5.2) and (5.7.4).

PROBLEMS

1. (a) Let I = ]a, h[ be an open interval in R, and let f be a real function defined and continuous in I. Suppose there is an at most denunierablesubset D o f l such that,foreach t E I - D,fis iticreusbig to the right at the point t , which means that there is an interval [ t , t + h ] ( h > 0) such that f ( t ) < f ( t ’ ) for t < t’ < t + h. Show that f is increasing in 1 (apply the same kind of argument as in (8.5.1)). Same conclusion when f i s only continuous on the left at each / E 1 (i.e. f ( t - ) = f ( t ) ) , but D = @.

5 THE MEAN VALUE THEOREM

(b) For each number t E J = [0, I [, let

m

161

aJ2" be the unique "dyadic" development

"=O

of t such that each a, is either 0 or I , and there is no index m such that a. n 2 m (see Section 4.2, Problem 2). Letf(t)

00

= Ca,/4". Show

=

1 for all

thatfis continuous on the

fl=O

2.

3.

right at every point t E J (i.e.f(t+) = f ( r ) ) , is not constant in any subinterval of J having more than one point, and that it has at every point t E J a derivative on the right, equal to 0. Show that the conclusion of (8.5.1) is still true if it is only supposed that f and q~ have both a derivative on the right at every point [ of I - D (fl being excepted), and that llfx3Il p m . Let f be a real continuous function defined in a compact interval [ a ,p], and having a derivative on the right at every point of ] a ,![. Let m and M be the g.1.b. and 1.u.b. of fi in la, P[. (a) Showthat iffisnotamappingt +ht p,thesetofallnumbers(f(x) - f ( y ) ) / ( x - y ) when x and y are arbitrary numbers in [ a , p] such that x # y , is identical to ]m,M[. (By suitable substraction of a function of the form t +ht f p, reduce the problem to showing that if fi(y)f,'(S) < 0 with a < y < 6 < fl, there are two distinct points in the interval ] a ,fit, wheref takes the same value.) (b) Show that if in additionfhas also a derivative on the left at every point of ] a ,p[, then the g.1.b. (resp 1.u.b.) of f,'andf,' in ] a ,p[ are the same. (c) Deduce from (b) that iff has a derivative at every point of ] a ,PI, the image by f ' of any interval contained in ] a ,/3[ is connected (see (3.19.1)). In the interval I = [- 1, + I ] of R , letfbe the mapping of I into R2defined as [allows: f ( t ) = (0,O)if - 1 < t < O ; f ( t ) = ( t 2 sin(l/t), t Z cos(l/t)) if 0 < t < 1. Show that f h a s a derivative at every point of 1- I , + I [, but that the image of that interval b y f is not connected. Extend (8.5.4) whenfis only supposed to be quasi-differentiable (Section 8.4, Problem 4) at every point of S, and f' stands for the quasi-derivative. Suppose F is a real Hilbert space. Deduce (8.5.1) from the same theorem for real functions g , by applying it to the real valued functions 6+ (f(5) 1 a), where a E F. (This method can in fact be applied to any Banach space, and even to more general classes of topological vector spaces; see [ 6 ] in the Bibliography.) Let I = [a, b ] be a compact interval in R not reduced to a single point, f a continuous mapping of 1 in a Banach space E. Suppose the derivative on the rightf,' exists at every point of 1 - D, where D is an at most denumerable subset of I containing b. Show that thereexistsa point 6 E I - D such that l\f(b) - f ( a ) l l < Ilf,'(t)l\(b - a).(Onemayassume that k = llf(b) -f(a)ll/(b - a) is not 0. Use contradiction, assuming that llf,'(t)ll < k for every f E 1 - D; then there would be a point x o E I - D and an h > 0 such that lIf(x0 h) -f(xo)ll< k h ; obtain a contradiction by applying Problem 2 to each of the intervals [a, x o ] and [xo h, b].) A subset A of a real vector space E is called convex if, for any pair of points x , y in A, the closed segment of extremities x , y (set of all points hx ( I - h)y for 0 < h < 1) is contained in A. In order that A be convex, a necessary and sufficient condition is that b the intersection of A and of a line, image of R by an affine mapping q~ : 6 +a6 (where a # 0 and b are vectors of E), be the image by 9 of an interval in R. A mappingfof the convex set A c E into R is called convex if, in the vector space E X R, the setDf of all pairs ( x , [ ) such that X E A and 5 >f(x), is convex. It is equivalent to say that for any pair of points x , y of A and every number h such that 0 < h G 1,

+

4.

5. 6.

7.

+

8.

+

+

+

f(Xx

+(I

- X)y)

< hf(x)+ (1

-

h)f(v).

Vlll

162

DIFFERENTIAL CALCULUS

In order that f be convex in A, a necessary and sufficient condition is that, for any affine mapping rp: (-.a( + h of R into E (with a Z 0) such that rp(R) n A # 0, the mappingfo IJC be convex in the interval ?-‘(A) c R. Let I 3 R be an interva1,fa real function defined in 1. In order thatf be convex in 1 it is necessary and sufficient that, for every a E I , the function t ( f ( t )- f ( a ) ) / ( t - a ) be increasing in 1 n C{a}.Conclude from that result that at every point a interior to I , f is continuous, and has both a derivative on the right and a derivative on the left; furthermore, if a < b are two points interior to I , we have --f

Conversely, if I is an open interval, f is continuous in I , has at every point a derivative on the right, and&’ is increasing in I , thenfis convex in I (use contradiction, and problem 2). I f A is convex set in a real vector space E, a mapping f of A into R is called sfricfly conuex if, for every pair of distinct points x , y in A and every number h such that O < h i 1, one has

f(Ax

+ (1

- A)Y)

<

+ (I

- h)f(Y>.

I f A = I is an open interval in R, in order t h a t f b e strictly convex in 1,anecessaryand sufficient condition is that& be strictly increasing in I .

6. APPLICATIONS OF T H E M E A N V A L U E T H E O R E M

Let A be an open connected subset of a Banach space E, f a continuous mapping of A into a Banach space F ; i f f has a derivative equal to 0 at ewry point of A, then J’ is a constant.

(8.6.1)

Let xo be a point of A, and let B be the set of points x E A such that . f ( x )=f(xo). B is closed with respect to A (3.15.1); on ,the other hand, if X E B and if U is an open ball of center x contained in A, then U contains the segment joining x to any of its points y , hence by (8.5.4) f’(y) =f(x) =.f(xo).This shows that B is also open with respect to A, hence equal to A by assumption (Section 3.19). Better results are available, using (8.5.2): for instance, if E = R and A is an interval in R, it is only necessary to assume that the derivative of f exists and is 0 except at the points of a denumerable set.

Let E, F be rwo Banach spaces, f a diferentiahle mapping into F of an open neighborhood A of a segment S joining tnio points a, b. Then, f o r each xo E A, ,i,e have

(8.6.2)

llf(b) - f ( a > - f ’ ( x o )

. ( b - a>II G Ilb - 4 . S U P lIS’(x> -f‘(xo)ll. xss

6 APPLICATIONS OF THE MEAN VALUE THEOREM 163

Apply (8.5.4) to the mapping x-f

(4- f ’(x,) . x

whose derivative is t + ( f ’(x) -f ’(x,) . t by (8.2.2) and (8.1.3).

(8.6.3) Let A be an open connected subset in a Banach space E, (f,)a sequence of differentiable mappings of A into a Banach space F. Suppose that: (1) there exists one point x, E A such that the sequence ( f , ( x , ) ) converges in F; (2)f o r every point a E A, there is a ball B(a) of center a contained in A and such that in B(a) the sequence (f,’) converges uniformly. Then f o r each a E A, the sequence (f,)converges uniformly in B(a); moreover, if, f o r each ~ E A , f ( x ) = lim f,(x) and g(x) = lim f ; ( x ) , then g(x) =f ‘ ( x ) f o r each x E A. n-r m

n-+ m

Let r be the radius of B(a); then by (8.5.4), for any point x E B(a), we have ilfn(x) -fm(x) - (fn(a> -fm(a>>ll G Ilx

- 41 . SUP IlfA(z1 -fh(z)ll IE

B(a)

(8.6.3.1) As the sequence ( f ; ) is uniformly convergent in B(a), and F is complete, this proves that if the sequence ( f , ( x ) ) is convergent at any point of B(a), it is also convergent at every point of B(a), and in fact uniformly convergent in B(a). This result first shows that the set U of the points x such that ( f n ( x ) ) is a convergent sequence, is both open and closed i n A; as it is not empty by assumption, and A is connected, U = A. We finally prove g is the derivative o f f : given E > 0, there is by assumption an integer no such that for n > n o , m > n o , 11 f,’(z) -fi(z)II < E/r for every z E B(a), and moreover llg(a) -f,’(a)ll < E ; letting m tend to + co in (8.6.3.1), we see that, for n 3 no and x E B(a), we have

IlfW - f ( 4- ( f , ( x ) -f,(a>>ll < EIIX - all. On the other hand, for any n 2 n o , there is r’ d r such that, for IIx - all G r’, we have IIf,(x) - f , ( a ) -f,’(a) . (x - a)ll < E /Ix - all ; using (5.7.4), we finally see that for IIx - all < r‘, we have

iifw- f ( a ) - g(a). (x - a)ii G 3

4 - ~ail

which proves that f ’ ( a ) exists and is equal to g(a). Q.E.D. Again, we can state better results when E = R and A is an interval in R :

I64

Vlll

DIFFERENTIAL CALCULUS

(8.6.4) Let (9,) be a sequence of mappings of an interval I c R into F, and suppose that, f o r each n, g,({) is the derivative of a continuous function j , except f o r the points 5 of a denumerable subset D, c I. Suppose in addition that: (1) there exists a point toE I such that the sequence (h(t0)) converges in F; (2)f o r every point E I, there is a neighborhood B(c) Iiith respect to I such that in B ( [ ) the sequence (g,) converges uniformly. Then f o r each 5 E A, the sequence (f,)converges uniformly in B(c); and i f Ice put f,(<)= lim f,(Q

c

and g ( { ) = lim g,({), then at every point of A not in n+ 00

u

n+ a,

D,, f’({)= g(5).

n

The proof repeats that of (8.6.3), using (8.5.2) instead of (8.5.4). (8.6.3) yields in particular: (8.6.5) Let A be an open connected subset in a Banach space, (u,) a sequence of differentiable mappings of A into a Banach space F. r f for every a E A, there is a ball B(a) of center a contained in A and such that the series (u;) is uniformly convergent in B(a), and i f there exists a point xo E A such that the series (u,(x,)) is convergent, then f o r each a E A, the series (u,) is unijormly m

convergent in B(a), and its sum s ( x ) has a derivative equal to every x E A.

ui(x) at n=O

PROBLEMS

Let f, g be two real valued differentiable functions defined in an open interval 1 c R. It is supposed that f ( t ) > 0, g(t) > O , f ’ ( f ) > 0 and g ’ ( t ) > 0 in 1. Show that if the function f ’ / g ’ is strictly increasing in I, either f / g is strictly increasing in I, or there exists c E I such that f / g is strictly decreasing for t < c and strictly increasing for t > c. (Prove that if f’(s)/g’(s) < f ( s ) / g ( s ) , then for any t < s, f’(t)/g’(f)< f ( t ) / g ( t ) . ) Apply to the function tan t tan u 1

U

t tan t - a tan a

in the interval ]a,n/2[. (a) Let I be an open interval in R,xo E R one of its extremitiesJa continuous mapping of I into a Banach space E. Suppose there is a denumerable subset D of I such that at each point of I - D,fhas a derivative on the right. In order thatf;(t) have a limit when t tends to xo in I - D, a necessary and sufficient condition is that ( f ( r ) - f ( s ) ) / ( f - s) have a limit when the pair (s, t ) tends to (xo, xo) in the set defined by s E I, / E I, s # t . Both limits are then the same; if c is their common value, show thatf( f ) has a limit in E when t tends to xo in I, and that i f f is extended by continuity to I u {xo} (3.15.5), ( f ( t ) - f ( x o ) ) / ( t - xo) tends to c when t tends to xo in I. (Use the mean value theorem and Cauchy’s criterion.)

7 PRIMITIVES AND INTEGRALS

165

(b) Show that at every point t E I D where fd is continuous on the left, f has a derivative on the left. If at t c I - D , f ; is continuous,fhas a derivative at the point t. (Use (a).) Let f be a differentiable mapping of an open subset A of E into F (E, F Banach spaces). (a) In order that f ’ be continuous at x o , a necessary and sufficient condition is that, for any E > 0, there exist 6 > 0 such that the relations Ilsll < 6, lltll < 6 imply ~

IImo

+S)

-f(xo

+ t ) -f’(xo)

(s - t)/l < EIIS - f l l .

‘

(b) In order that f ’ be uniformly continuous in A, a necessary condition is that, for any E > 0, there exist 6 > 0 such that the relations IIsIl < 6, x E A, x 5s E A for 0 < 6 < I imply llf(x s) - f ( x ) - f ’ ( x ) . s/1 < EIISII. The condition is sufficient when A is convex (Section 8.5, Problem 8). Letfbe a continuous mapping of a compact interval I c R into R , having a continuous derivative in 1. Let S be the set of points t E I such that f ’ ( f ) = 0. Show that for any E > 0, there exist a sequence (r.) of numbers > O such that 1 r. < E and that the set

+

+

m

“=O

f ( S ) is contained in a denumerable union of intervals J,, such that 6(JJ < r , . (For any a > 0, consider the open subset U, of I consisting of the points t where if’(t)i < a ; use (3.19.6) and the mean value theorem.) Let f b e a continuous mapping of an interval 1 c R into C , such that f ( t ) # 0 in I and and that &(t) exists in the complement of a denumerable subset D of 1. In order that I f 1 be an increasing function in 1. show that a necessary and sufficient condition is that 6.

.?R(fi(t)/f(t)) 2 0 in I - D. Let E, F be two Banach spaces, A an open subset of E, B a closed subset of the subspace A, whose interior is empty and such that any segment in E which is not contained in B has an at most denumerable intersection with B. Letfbe a continuously differentiable mapping of A - B into F, and suppose that at each point b E B, the limit of f’(x) with respect to A - B exists. Show that fcan be extended by continuity to a continuously differentiable mapping3 of A into F (same method as in Problem 2(a)).

7. PRIMITIVES A N D INTEGRALS

Let f be a mapping of an interval I c R into a Banach space F. We say that a continuous mapping g of 1 into F is a primitive off in I if there exists a denumerable set D c I such that, for any 4 E I - D, g is differentiable at 4 and g’(4) = f ( O .

(8.7.1) I f g l , gz m e t1t-o primitives o f f in 1, then g1 - g 2 is constant in I. This follows at once from the remark following (8.6.1). Any interval I in R (not reduced to a point) is the union of an increasing sequence of compact intervals J,; to check that a function f defined in I has a primitive, it is only necessary to do so for the restriction off to each

166

Vlll

DIFFERENTIAL CALCULUS

of the J,: for if tois an interior point of J,, and if, for each 12, g , is the primitive in J, of the restriction off to J,, such that g,((,) = 0 (which is uniquely determined by (8.7.1)), then the restriction of gn+l to J, is a primitive of f i n J, vanishing at l o ,hence equal to g , . We can therefore define the mapping g of I into F as equal to g, in each of the J,, and it is obvious that g is a primitive off in 1.

(8.7.2) Let I be an interval of R; any regulated mapping of I into F (Section 7.6) (and in particular any cot~tinuousmapping into F, or - when F = R -any monotone function) has a primitive in I. From the preceding remarks, it follows that we can assume I is compact. Then, from (8.6.4) and (7.6.1) it follows that we need only prove the theorem for step-jiinctions. Suppose f is a step-function, (Ai)Osisn an increasing sequence of points in I = [a, p] such that I , = CI, A,,= p and f ( ( ) is equal to a constant ci in ]Ai,Ai+l[ (0 < i < n - 1). Then if we define (0 < i < n - I), we have g(5) = g such that in each interval [Ai,

ci(5 - Ai) +

i-1

1

k=O

Ck(&+,

- Ak), it is readily verified that g is a primitive off.

A primitive of a step-function is also called a piecewise linear function. For a continuous function, we have furthermore : (8.7.3) I f g is a primitive of a continuous mapping f of I into F, then g has at every point 4 E I a derivative with respect to I equal to f (5). For it follows from (8.5.2) that for every interval

l l d 5 + 5) - S(t) -f(05II for 0 < 5 < A, and sup assumption.

OQqC.2

11 f(5

Q

5o sSUP llf(5 qsa

+ u) - f(5)Il

[5, 5 + A] c I

+

17)

-f(OII

is arbitrarily small with A, by

If g is any primitive of a regulated function f , the difference g ( p ) - g(a), for any two points of I, is independent of the particular primitive g which is considered, owing to (8.7.1); it is written f(() d ( , and called the integral off between CI and p. Any formal rule of derivation can be translated into that notation and yields a corresponding formula of “integral calculus ”; we only write explicitly the three most important ones; for convenience, if g is a primitive of a regulated function5 we write g’ instead off, although

7

PRIMITIVES AND INTEGRALS

167

g does not have in general a derivative everywhere, and when the derivative exists it may fail to be equal to f (at the points of a denumerable set): (8.7.4) (“Change of variables”) Let q be a real-tlalued primitive of a regulated function defined in an interval I; let f be any regulated function defined in an interval J 3 ~ ( 1 ) ; then, if either f is continuous or cp is monotone, f o r any two points u, fl of I, we have ju’f(B(5:))Q.(O d5: =

S*”

(PCU)

f(i>di.

The only point to check is that f((p(l))(p’(l) is a regulated function, which follows at once from the assumptions and from the definition of a regulated function (Section 7.6); then, if g is a primitive ofS, both sides of the formula are equal to g(cp(p)) - g(q(u)), due to (8.4.1). (8.7.5) (“Integration by parts”) L e t f , g be primitives of regulated functions defined in an interval I, and taking their values in two Banach spaces E, F respectively; and let (x, y ) -+ [x * y ] be a continuous bilinear mapping of E x F into a Banach space G; then, f o r any points a, B of I

ja’If(t). ~’(511d t

= “9

Ju’

s(P11- If(@). s(u>l- Cf’(4) . dt)l d5.

Again, the only point to check is that [f . g’] and [f ‘ . g ] are regulated functions, and then the formula follows from (8.1.4) and (8.4.1). 8.7.6) Let f be a regulated mapping of I into a Banach space F, and let u be any continuous linear mapping of F into a Banach space G. Then

This follows from (8.4.1) and (8.1.3). The translation in terms of integrals of the mean value theorem reads: (8.7.7) For any regulated function f in a compact interval,

168

Vlll DIFFERENTIAL CALCULUS

Here again, to apply (8.5.1) we have only to verify that 5 --f 1) f(t)11 isregulated. Finally, we express for integrals results corresponding to (8.6.4) and (8.6.5): (8.7.8) Zfa sequence (g,) of regulated functions, defined in a compact interval I = [a, p], converges uniformly in I to g, then the sequence (japg,(t)d t ) con-

verges to Japg(t)d t . (Remember g is regulated by (7.6.1).)

(8.7.9) If a series (u,) of regulated functions, defined in a compact interval

I = [a, p], is normally convergent (Section 7.1) in I, then, if u = the series of

general term

s.”

m

1 u,,

n=O

u,(t) d< is absolutely convergent, and

The absolute convergence follows at once from the assumption and the mean value theorem (8.7.7). (8.7.10) Remark. Due to (8.6.4) and the proof of (7.6.1), for any regulated function f defined in [a, p], and for any E > 0, there is an increasing sequence a

= x g

such that

< to < x 1 < t l <

lls.BI(x)

< x k < fk
* * *

dx -

n- 1

-f(fk)(Xk+ 1 k=O

- xk)

I/ <

’ * ’


&.

I f f is continuous, one may (due to (3.16.5)) take all numbers equal to ( p - a)/n, and t k = xk (see Problem 1).

Xk+l

- x,

PROBLEMS

1. Let f be a regulated function defined in a compact interval I c R. Show that for any E > O , there is a number 6 > 0 such that for any increasing sequence X o < f o ~ X i < , “ ‘ ~ X k < f , , < x k + l <“’
(“Riemann sums”; consider first the case in which f i s a step-function).

7 PRIMITIVES A N D INTEGRALS

169

2. (a) Let f be a regulated function defined in a compact interval I = [a, b]. Show that for any E > 0, there exists a continuous function g defined in I and such that Jabllf(t) - g(t) /I dt G E.

(b) Supposeftakes its values in E; let h be a regulated function defined in I and taking its values in F, and let ( x , y ) + [ x . y ] be a continuous bilinear mapping of E x F into G (E, F, G Banach spaces). Show that

(c) Show that n- m

(d) Let u be a primitive off, and suppose u(I) is contained in a ball B c E. Show that if g is a monotone function in I, then there exists a number c E B such that

Jn>(Mf) dt = ( 4 b ) - cMb)

+ (c - u(a))g(a).

In particular, i f f is a real regulated function, there exists s E I such that

J a > ( M r ) dt = g(a)J:f(r) dt

+ d b ) l f ( r )dt

("the second mean value theorem "). (For all these properties, use the same method as in Problem 1.) 3. Let f be a regulated function defined in a compact interval I = [a, b]. For any integer n > 0 and any integer k such that 0 < k < n, let xk = a k((b - a)/n);let

+

(a) Supposefhas a continuous derivative in I. Show that b-a lim nr(n) = -( f ( b )- f ( a ) ) . n-m 2 (Write r(n) =

"-1

k=O

1

Xk+l (f(xk+l)

- f ( f ) ) dt; use the mean value theorem and Problem 1.)

~t

(b) Supposefis an increasing real function in I; show that

(c) Give an example of an increasing continuous function f i n I such that nr(n) does not tend to ((b- a ) / 2 ) ( f ( b ) - f ( a ) ) when n tends to +m. (Take for f the limit of a sequence (f.)of increasing continuous piecewise linear functions, satisfying the conditions

and f.+i(a+k

b-a

7) =f.(a+

k

b-a

7 ) for

0

< k < 2".)

Vlll DIFFERENTIAL CALCULUS

170

4. Show that, when n tends to +a,the polynomial

converges uniformly to - 1 in any interval [- 1, inanyinterval Let g.(x)

+I](& > Oarbitrary; use the inequality

1:

=

and converges uniformly to

jO1(i dt >/o’(l

+1

- 1)” d r ) .

t2)n

f.(t) dt; show that the polynomial gn converges uniformly to the function

in [- 1, +I], obtaining thus a new proof of (7.3.1.3)). Letfbe a continuous mapping of an interval [xo , +a[into a Banach space E, such that for each h > 0, lim ( f ( x h) - f ( x ) ) = 0.

1x1

5.

[E,

-E]

+

+

x-+m

+

(a) Show that f ( x h) - f ( x ) converges uniformly to 0 when x tends to co and h remains in a compact interval K = [a, b l c [0, 00 [ (i.e., for every E > 0 there exists A > 0 such that x 3 A implies Ilf(x h) -f(x)ll f E for every h E K). (Use contradiction: suppose there is a sequence (x,) such that lim x. = 00, and a sequence (A,) of

+

+

+

+

n-m

points of K such that Ilf(x. h,) --f(x”)Il > a > 0, for every n. Observe that there is a neighborhood J. of h. in K such that Ilf(x. h) - f ( x . ) 11 > a for any h E J, Define by induction a decreasing sequence of closed intervals Ik c K, and a subsequence (xnk)of (x.) such that Ilf(x., p) -f(xnk)ll 3 a/3 for every p E I k ; to define I k + , when 1, is known, observe that if 6, is the length of Ik, and q an integer such that q6, > b - a, then Ilf(x 6,) -f(x)II < a/3q as soon as x is large enough.)

+

.

+

+

(b) Deduce from (a) that

lirn ( j I + > ( t ) d t - f ( x )

= 0,

and conclude that

x-+m

lim f ( x ) / x = 0. X-tW

6. (a) Show that there exists a differentiable real function f (resp. g) defined in

R and such that f ’ ( t ) = sin(l/t) (resp. g’(t) = cos(l/t)) for t # 0 and f’(0)= 0 (resp. g‘(0) = 0). (Consider the derivatives of the functions t 2 cos(l/t) and t Z sin(l/t).) The functions f’ and g‘ are not regulated. (b) Let P(t, u , u) be a polynomial in u and u with coefficientswhich are continuous real functions of t in an open interval I c R containing 0. Show that there exists a differentiable functionfdefined in I. such thatf’(t) = P(t, sin(l/t), cos(l/t)) for t # 0 (express monomials in sin(l/t) and cos(l/r) as linear combinations of terms of the form sin(k/r) or cos(k/t), and use (a)), What is the value of f’(O)? Show that one may have f’(0)# P(0, 0,O). (c) Show that there exists a differentiable functionfdefined in [-1, + I ] and such that f ’ ( t ) = sin(l/sin(l/t)) at every point t other than 0 and the points l/nn (n positive or negative integer). (In the neighborhood of t = l / n n , write t = (nr arc sin u ) - l and use (b) to prove the existence off’(l/nr); furthermore, show that there is a constant a > 0, independent of n, such that

+

(/

Z/(zn-l)n

Z/(Zn+ 1 ) n

sin(l/sin(l/t)) dt

I<

a/n3

for every integer n > 0; consider then the function g ( t ) = lim I*sin(l/sin(l/s)) ds c-0

c

for t > 0, and define f similarly for t < 0.)

8 APPLICATION: THE NUMBER e 171

7.

Let I = [0, 1 [ and let E be the vector space of regulated complex functions defined in I, bounded and continuous on the right (i.e. f ( f + ) = f ( f ) for t E I). (a) Show that on E, (f1 g)

=I f(f)s(t) +1

df is a nondegenerate positive hermitian form

-1

(see (8.5.3)). Prove that the prehilbert space E thus defined is not complete (use the fact that the function equal to sin(l/f) for f > 0, to 0 for t = 0, is not in E). (b) Define the sequence (f.)of elements of E in the following way: (1) fo is the constant 1 ; (2) for each integer n > 0, let rn be the largest integer such that 2" < n, and let 2k n = 2'" k ; f, is taken as equal to 2"'' for < f < 2k 1 to -2"" for

+ 2k + 2 5 2"+1
2"++"

2m+'

and to 0 for all other values of f in I.

Prove that in the prehilbert space E, (f.)is an orthonormal system (the "Haar orthonormal system "). (c) For each n > 0, let V, be the supspace of E generated by the& of indices k < n. Show that there is a decomposition of I into n 1 intervals of type [a, without common points, such that, in each of these intervals, every function belonging to V. is constant; conversely, every function having that property belongs to V. (consider the dimension of the vector subspace of E generated by these functions). (d) Let g be an arbitrary function of E, h its orthogonal projection (Section 6.3) on V. ; show that in each of the intervals [a,/?[in which all the functions of V. are constant,

+

1

p[

I

0

h(t) = - g(u) du. /?-a D (e) Show, by using (d), that for any function g E E which is confinuous in I, the series of general term (g I f . ) f . ( t )is uniformly convergent in I and that its sum is equal to g ( t ) . Conclude from that result that (f.)is a total orthonormal system in E.

8. Let f b e a regulated real valued function in a compact interval 1 = [a, 61; let

dt =

E

> 0, there is a real valued continuous function g in I, such that

1 in I, and that

dt 3 c - E . (Reduce the problem to thecasein whichf

c. Show that for any 1g(f)l S

II f ( f ) [

is a step function.)

8. APPLICATION: T H E N U M B E R e

For any number a > 0, the function x + ax is continuous in R (Section 4.3), hence the function g(x) a' dt is defined and differentiable in R,with

=jx 0

g'(x) = deverywhere. Nowwehaveg(x + 1)

But by (8.7.4),

sx+'.' ax" dt

=I

x+ 1

a'dt =Joxa'dt +JxX+'atdt.

0

du = u x ~ o ' adu; u as ax 2 inf(a, 1) for

X

a" du is

> 0 by (8.5.3), hence we can write ax = c - ' ( g ( x

+ 1) .- g(x))

172

Vlll

DIFFERENTIAL CALCULUS

and therefore a" is d/ferentiable in R, and D(a") = cp(a) . a", where cp(a) # 0 if a # 1. Suppose a # 1, and let b be any number 1 0 ; we can write b"

= axlogab

and therefore, by (8.4.1)

cp(b) . b" = log, b . cp(a) . b", in other words

0)= c p ( 4 log, 6 . There is therefore one and only one number e > 0 such that cp(e) = I , namely e = as D(e") = ex > 0, ex is strictly increasing (by (8.5.3)), and hence e = e' > e0 = 1. The function ex is also written exp(x) or exp x. The function log, x is written log x and it follows from (8.2.3)and (4.2.2) that D(log x) = I/x for x > 0. Furthermore D(a") = log a * a".

PROBLEM

Study the variation of the functions (1

+

!-)"".

(1

+ $)Y

(1 +!)(I

+ ;):

(I

+

y+'

for x > 0 , p being a fixed arbitrary positive number; find their limits when x tends to

+to.

9. PARTIAL DERIVATIVES

Let f be a differentiable mapping of an open subset A of a Banach space E into a Banach space F; Dfis then a mapping of A into 9 ( E ; F). We say that f is continuously diferentiable in A if Dj i s continuous in A. Suppose now E = E, x E, . For each point (al, a z )E A we can consider the partial mappings x1 +f ( x , , a z ) and x, +f(a,, xz) of open subsets of El and E, respectively into F. We say that at (a,, a,), f is d/ferentiab/e with respect to the first (resp. second) variable if the partial mapping x1 +f(xl, a 2 ) (resp. x2 +f ( a , , xz)) is differentiable at a, (resp. a z ) ; the derivative of that mapping, which is an element of -%'(El ;F) (resp. 9 ( E z ; F)) is called the partial derivative off at ( a l , a,) with respect to the first (resp. second) variable, and written D,f(a,, a,) (resp. D,f(a,, a,)).

PARTIAL DERIVATIVES

9

173

(8.9.1) Let f be a continuous mapping of an open subset A of El x E, into F. In order that f be continuously differentiable in A, a necessary and suficient condition is that f be differentiable at each point with respect to the first and the second variable, and that the mappings (x, , x,) -+ D, f ( X I , x,) and (x,, x,) -+ D,f(x,, x2) (of A into 2’(El; F) and 9 ( E 2 ; F) respectioely) be continuous in A. Then, at each point (x,, x,) of A, the derivative off is given by (8.9.1.1)

Df(x1,

~

2

)(11, t 2 )

=Dif(xi,

~ 2 ) ti.

+ Dzf(x1,

~ 2 ) t. 2 .

(a) Necessity The mapping x, -)f(x,, a,) is obtained by composing f and the mapping x1 -+ (x,, a,) of El into El x E2 , the derivative of this second mapping being r, - + ( t , , 0) by (8.1.2), (8.1.3), and (8.1.5). Then by (8.2.1), x1 f(x,, a,) has at (a,, a,) a derivative equal to t , -+ Df(a,, a,) . (tl, 0). If we call i, (resp. i,) the natural injection t , .+ ( t , , 0) (resp. t2 + (0, t , ) ) , which is a constant element of 2’(El; E, x E2) (resp. 2’(E2; E, x E,)), we therefore see that D, f ( a l ,a,) = Df (al, a,) i,, and similarly D2f (al, a,) = Df(a,, a2) i, (all this is valid iff is simply supposed to be differentiable in A). As the mapping ( 0 , u ) -+ v u of 9(El x E,; F) x 9( E l ; El x E,) into 9 ( E l ; F) is continuous ((5.7.5) and (5.5.1)), the continuity of D, f and D, f follows from that of Df; finally, as ( t , , t , ) = il(tl) + i2(t2),we have (8.9.1 .I). (b) Suficiency Write -+

0

0

0

f ( 0 , + 4 > a2 + f 2 ) - f (a19 a,) = (f(a1

Given

E

+ t , , 0 2 + t 2 ) -f(a, + t , , 0 2 ) ) + ( f b l + t , , a,)

-f(a,, a,)).

> 0, there is, by assumption, an r > 0 such that, for IItlll d r IIf(a1

+ t , , a2) -f(a,,

02)

- D,fta,,a2), fill

E

Iltlll.

On the other hand, we have in a ball B of center (al, a,) contained in A, by (8.6.2) lIf(a1

+ t , , a2 + 12) -f(a1 + t l , 0 2 ) - D,f(a1 + t l , 0 2 )

*

t2II

The continuity of the mapping D, f therefore implies that there is r‘ > 0 such that for /lt,Ij < r’ and Ilt,II < r‘, we have IIf(a,

+ t,, a, +

-f(a,

+ t , , 0,) - D2f(al + I , ,

0,)

- t,Il

and on the other hand

I/ D2f (01 + t17a,) - D2f (al, a,)ll


hence, by (5.7.4) lIDzf(a1 + t ~02) ,

t2

- D,f(ai,

02)

. t ~ l ld E llt,ll.

G E IIt211

174

DIFFERENTIAL CALCULUS

Vlll

Finally, for sup((lt,I(, IIt211) < inf(r, r') we have IIf(a1 + t,, a2

+ tz) - f ( a , ,

a,)

- D,f(a,, a,) * tl - Dzf(a1, 6 ) .hIl

G 4 E SUP(IItl I1 I1t z Ill 9

which proves (8.9.1.1); the continuity of Df follows from the fact that (8.9.1.1) can be written Df= D, f 0 prl D, f 0 pr, and from (5.7.5). Theorem (8.9.1) can be immediately generalized to a product of n Banach spaces by induction on n ; if we combine that result with (8.2.1), we obtain

+

(8.9.2)

Let f be a continuously direrentiable mapping of an open subset

A of E =

n n

i=l

Ej into F, and, for each i, let g i be a continuously differenti-

able mapping of an open subset B of a Banach space G into E i , such that (g,(z),. . ,gn(z))E A for each z E B. Then the composed mapping h = f 0 ( g l , . . . , gn) is continuously differentiable in B, and we have

.

=c(Dk f(gl(z), . n

Dh(z)

k= 1

* * 9

gn(z)))

Dgk(Z)*

PROBLEMS

1. Let E, F be two Banach spaces,f a continuous mapping of an open subset A of E into F. Suppose that for each x E A, there is an element u ( x ) E Y(E; F) such that, for every vector y E E, the limit of ( f ( x ty) - f ( x ) ) / f when t # 0 tends to 0 in R,exists and is equal to u(x) . y. Suppose in addition that x + u(x) is a continuous mapping of A into Y(E; F). Show that f is then continuously differentiable in A and that u(x) = D f ( x ) for every x E A. (Apply the mean value theorem to the function f + f ( x f y )for t E [0, 1 I.) 2. Let E be the space (co) of Banach (Section 5.3, Problem 5 ) ; let F be the complex Banach space (co) i(co),consisting of all sequences z = (,O of complex numbers such that lim 5. = 0, with the norm llzll= sup 15.1. We denote by Fo the real Banach space

+

+

+

"

n- m

underlying F (Section 5.1). Let I C R be an open interval containing 0, and for each integer n 3 0, let f. be a continuous mapping of I into C such that the condition lim I, = 0 implies lim f&) = 0; this defines a mappingf: (6")*(fa((")) of E into Fo n-m

n-m

.

(a) Suppose f i s continuous in a neighborhood of 0. In order that f be quasi-differentiable at the point 0 (Section 8.4, Problem 4), it is necessary and sufficient that for each n the derivative f.'(O) exist and that there exist two numbers A > 0 and 6 > 0 such that the relation It1 < 6 implies If.(t) -f.(O)I < Alrl for every n ; this implies that S U P lL(0)l < +a. (b) In order that f be differentiable at 0, it is necessary and sufficient that for every E > 0, there is a 6 > 0 such that the relation It1 G 6 implies Ifn(t)-fn(O) -K(O)tI G E It1 for every n.

10 JACOBIANS 175

(c) In order that the derivativef' exist in a neighborhood of 0 in E and be continuous at 0, a necessary and sufficient condition is that there exist a neighborhood J c I of 0 such that: (1) eachf,' exists in J ; (2) sup Ifk(0)l < + a ;(3) the sequence(f.')isequiconn

tinuous a t the point 0 (Section 7.5). (See Section 8.6, Problem 3.) (d) Let f . ( t ) = e""/n for every n 2 1,Yo(()= I . Show that f is quasi-differentiable at every point x E E; if u(x) is the quasi-derivative o f f at the point x , show that the mapping ( x , y ) u(x) . y of E x E into Fo is continuous, but that f i s not differentiable at any point of E. Let f be a continuous mapping of an open set A of a Banach space E into a Banach space F. Suppose that for any x E A and any y E E, lim ( f ( x t y ) - f ( x ) ) / t = g ( x , y ) --f

+

c-o,r+o

exists in E. If, for y l E E, 1 < i < n, and xo E A, each of the mappings x + g ( x , y l ) is

+ + +

I:

continuous at xo , show that g ( x o , y1 yz . . . y.) = g(xo,y l ) (apply the mean 1=1 value theorem). Let E l , Ez , F be three Banach spaces,fa continuous mapping of an open subset A of El x E2 into F. In order that f be differentiable at ( a l , a 2 )E A, it is necessary and sufficient that: (1) Dlf(al, a 2 ) and D2f(al, a 2 ) exist; (2) for any E > 0, there exists 6 > 0 such that the relations IIfJ < 6, Ilt2i1< 6 imply Ilf(a1

+ t l , a , + t z ) -fh+ t l ,

0 2 ) -f(a1,

a2

+

t 2 ) +f@l,

aJil < E(llflll+ Iltzll).

Show that the secondconditionissatisfied if Dlf(al, az)exists and thereisaneighborhood V of (al, a 2 ) in El x E2 such that D2f exists in V and the mapping ( x I ,x 2 ) - + D 2 f ( x I x, z ) of V into P ( E 2 ; F) is continuous. Let f be the real function defined in R Z by f(x,y ) = (xy/r)sin(l/r) for (x, y ) # (0, O), with r = (x' Y ' ) ' ' ~ , and f ( 0 , O ) = 0. Show that Dlf and D 2 f exist at every point ( x , y ) E R2, and that the four mappings x Dlf(x, b), y Dlf(a, y), x D 2 f ( x , b), y --f D 2 f ( a ,y ) are continuous in R for any (a, b) E R2, but that f i s not differentiable at

+

--f

--f

--f

NO).

Let I be an interval in R , f a mapping of I' into a real Banach space E, such that, for any ( a l , ... , a,) E I,, each of the mappings x , + f ( a ~ ,... , u,.-I, x j . U J + I , . . ,a,) (1 < j < p ) is continuous and differentiable in I, and furthermore, the p functions D j f ( l < j < p ) arc bounded in 1'. Show thatfis continuous in I' (use the mean-value theorem).

.

10. JACOBIANS

We now specialize the general result (8.9.1) to the most important cases. 1. E = R" (resp. E = C"). Iff is a differentiable mapping of an open subset A of E into F, the partial derivative D,f(a,, . . . , a,) is identified to a vector of F (Section 8.4), and the derivative off is the mapping fl

((1,

cn)+

k=l

Dkf(al?

...)'%)lk.

If Dfis continuous, SO is each of the Dk$ Conversely, if each of the mappings DJexists and is continuous in A, then f is continuously differentiable in A. 2. E = R" and F = R" (resp. E = C" and F = C'"). Then we can write

176

Vlll

DIFFERENTIAL CALCULUS

f = (ql,. . . , cp), where the qi are scalar functions defined in E, and by (8.1 -5)f is continuously differentiable if and only if each of the 'pi is continuously differentiable; again, by case I , 'pi is continuously differentiable if and only if each of the partial derivatives D j q i (which is now a scalar function) exists and is continuous. Furthermore, the (total) derivative of f is the linear mapping (Cl,

with

*.*)

Cn)+(Vl,

. . . >V m )

n

Vi

=

C (Djcpi(E1, j=

* * * 3

an>>Cj ;

1

in other words, f',which is a linear mapping of R" into R" (resp. of C" into C"), corresponds to the matrix (Djqi(al, . . . , a,)), which is called the jacobian matrix o f f ( o r of cpl,. . . , cp), at (al, . . . , a,,). When m = n, the determinant of the jacobian (square) matrix off is called thejacobian off(or of cpl, . . ., cp,). Theorem (8.9.2) specializes to Let cpj (1 < j < m ) be m scalar functions, continuously diferentiable in an open subset A of R" (resp. C"); let $i (1 < i < p ) be p scalar functions, continuously diflerentiable in an open subset B of R" (resp. C") containing the image of A by (ql, .. ., cp;), then $ O,(x) = $i(cp,(x), ..., cp,(x)) for x E A and 1 < i < p , we have the relation

(8.10.1)

(Dk Oi) = (Dj II/i)(Dkq j ) between the jacobian matrices; in particular, when m = n = p , we have the relation det(D,Oi) = det(Dj$,) det(Dkcpj) between the jacobians.

a

We mention here the usual notations f;,(tl, . . . , t,,),-f ( t l , . . . , t,,),

at i

for Dif(tl, . . . , tn),which unfortunately lead to hopeless confusion when substitutions are made (what does f,'(y, x) or f J x , x ) mean?); the jacobian det(Djcpi(tl, . . . , t,)) is also written D(cpl, . . ., cp,)/D(t,, . . . , t,) or d(cp1, * * * cp")/a(tl, . . * , t"). 9

11. DERIVATIVE O F A N INTEGRAL DEPENDING O N A PARAMETER

(8.1 1.I)Let I = [cr, /3] c R be a compact interaal, E, F real Banach spaces, f a continuous mapping of I x A into F (A open subset of E). Theti g(z) = ff(t, z ) d( is continuous in A.

11 DERIVATIVE OF A N INTEGRAL DEPENDING ON A PARAMETER

177

Given E > 0 and zo E A, for any 5 E I, there is a neighborhood V(5) of 5 in I and a number r(5) > 0 such that for q E V(5) and ))z- zo))< r(5), 11 f ( q , z) -f(5, zo)II < E . Cover I with a finite number of neighborhoods V(ti), and let r = inf(r(5J). Then IIf ( 5 , z) -f ( 5 , zo)Il < E for llz - zoII < r and any 5 E I; hence, by (8.7.7) for llz - z0II < r. Q.E.D.

It&) - g(z0)II

< E(B - 4

(8.11.2) (Leibniz’s rule) With the same assumptions as in (8.11.1), suppose in addition that the partial derivative D2f with respect to the second variable exists and is continuous in I x A. Then g is continuously diflerentiable in A, and

(observe that both sides of that formula are in Y(E; F)). The same argument as in (8.51.I) applied to D,f, shows that given E > 0 and zo E A, there exists r > 0 such that IID, f ( 5 , z ) - D, f ( 5 , zo)I(< E for ))z- zo)I< r and any 5 E I; hence, by (8.6.2)

+ t ) -At, ZO) - D, f(5, zo) tll < E lltll for any 4 E I and any t such that lltll < r. By (8.7.7)we therefore have IIf(5,

20

*

Butby(8.7.6)and(5.7.4)wehaveJa’(D2f(t;,zo).t)d5 =(fD2f(5,zo)d5) . t for any t , and this ends the proof.

PROBLEMS

1. Let J C R be an open interval, E, F two Banach spaces, A an open subset of E, f a continuous mapping of J x A into F such that D,fexists and is continuous in J x A, 0: and fl two continuously differentiable mappings of A into J. Let

/*(,,”c, O(Z)

g(z) =

4d6.

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DIFFERENTIAL CALCULUS

Show that g is continuously differentiable in A, and that y’(z) is the linear mapping

2.

(apply (8.9.1) and (8.11.2)). Letf,g be two real valued regulated functions in a compact interval [a, b ] ,such thatfis decreasing in [a, b] and 0 < y ( t ) < 1. Show that

s,:

where A

=

s.”

g(1) dt.

1f

( t ) dt

< /abf(t)g(t)dt < JOa+h) dt

When is there equality? (Consider the integrals

Jba’+h”)f(t) dr, where h(y) =

1

g ( t ) df, as

s:

f ( t ) g ( t ) dt and

functions of y , and similarly for the other

inequality.) 3. Let the assumptions be the same as in Problem 1, except that CL and p are merely supposed to be continuous, but not necessarily differentiable, but in addition it is supposed that f(ar(z), z)= 0 and f(P(z), z ) = 0 for any z E A. Show that g(z) is continuously differentiable in A, and that g’(z)

L P(Z)

=

Dzf(E, z ) d[. (Use Bolzano’s theorem (3.19.8)

to prove that if 5 belongs to the interval of extremities p(zo) and p(z), there i3 z’ t A such that llz’ - zo/I< Ilz ZOII and [ = p(z’); if M is the 1.u.b. of llDzfll in a neighborhood of (/3(zo),zo), use the mean value theorem to show that Ilf([, z)ll < M Ilz - zoI/. 4. Let 1 [a, b ] ,A = [c, d ] be two compact intervals in R , f a mapping of I x A into a Banach space E, such that: ( I ) for every y E A, the function x + f ( x , y ) is regulated in I and for every x t 1, the function y -.f(x, y ) is regulated in A; ( 2 ) f is bounded in I x A ; (3) if D is the subset of I i( A consisting of the points (x, y ) where f is not continuous, then, for every xo E I (resp. every y o E A), the set of points y (resp. x) such that (xo,y ) E D (resp. (x,y o ) t D) is finite. ~

Jgb

(a) Show that the function g ( y ) = f ( t , y ) dt is continuous in A. (If

E

> 0 and

y o E A are given, show that there is a neighborhood V of y o in A and a finite number of intervals Jr c I (1 < k < n) such that the sum of the lengths of the Jr is < E and that, if

W =I -

u J, , f i s continuous in W x V; to prove that result, use the Borel-Lebesgue

k = l

theorem (3.17.6).) (b) Deduce from (a) that

/:lrv Jabf(x,Y ) dx = jabdxJcd f ( x , Y ) dy.

s: Jab

(Consider the two functions z + dy

f ( x , y ) dx and z -->

(Cf. Section 13.21). (c) Deduce from (b) that if a = c, b = d , then

(consider the function equal to f ( x , y ) for y

< x , to 0 for y > x).

y ) dy for z E A.)

12 HIGHER DERIVATIVES 5.

179

(a) Let f be a strictly increasing continuous function in an interval [0, a ] , such that

r’

f(0) = 0 ; let g be the inverse mapping, which is continuous and strictly increasing in the

.c,p < <

interval [O,f(a)].Show that

f ( t ) dt

=

(a - & I ) )

du (apply Problem 4 to the func-

tion equal to 1 for 0 < x a , 0 y < f(x), to 0 for 0 < x < a , f ( x ) < y < f(a)). (b) Show that if 0 < x < a and 0 < y < f(a), the following inequality holds

the two sides are equal if and only if y =f(x). (c) Peduce from (b) the inequalities xy G x xy

. log x + e Y - ’

< ax’

+b p

for x>O,

ysR;

for x>O,

y20, p>l,

q>l,

a > 0,

1

1

-+-=1, P Q

b > 0 and (pa)4(qb)p> 1.

12. HIGHER DERIVATIVES

Suppose f is a continuously differentiable mapping of an open subset A of a Banach space E into a Banach space F. Then Dfis a continuous mapping of A into the Banach space 9 ( E ; F). If that mapping is differentiable at a point xo E A (resp. in A), we say thatfis tiisice drfSerentiable at x, (resp. in A), and the derivative of Df at xo is called the second deriuatiue of f a t x,,, and writtenf”(x,) or D2f(xo). This is an element of 2(E; 9 ( E ; F)); but we have seen (5.7.8) that this last space is naturally identified with the space 9 ( E , E; F) (written Y2(E; F)) of continuous bilinear mappings of E x E into F: we recall that this is done by identifying u E 2 ( E ; 9 ( E ; F)) to the bilinear mapping (s, t ) ( u * s) t ; this last element will also be written u . (s, t ) . --f

(8.12.1) Suppose f is tii’ice diferentiuble at x,; then, for any jixed t E E, the derioative of the mapping x-)Df(x) . t of A into F, at the point x o , is s + D2f(x0) . (s, t ) .

If we observe that x -+ Df(x) . t is composed of the linear mapping u + u . t of 9(E; F) into F and of the mapping x Df(x) of E into Y (E; F) the result follows from (8.2.1) and (8.1.3). -)

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(8.12.2) If f is t,cice diferentiable at xo, then the bilinear mapping ( s , t ) --t D2f(xo) . (s, t ) is symmetric, in other rvords D2f(xO) . (s, t )

= D2f(xO)

Consider the function of the real variable

. ( t , $1.

5 in the interval [0, 11:

g(5) =f ( x o + 5s + 0 - f ( x o + 5s)

where s, t are such that llsll < f r , lltll < f r , the ball of center xo and radius r being contained in A. From (8.6.2) we get

and

II fYx0 + 5s) - f’(x0) - f”(xo> . (5s)/I

E

IIs I1

hence

lls’(5) - ( f ” ( X 0 ) . t >

*

sII G 2E llsll . (llsll

+ lltll)

and therefore

Ildl)- d o )

- (f”(X0) . t ) . sII

+ +

+

< 6.2 llsll (llsll + Iltll).

t ) -f(xo But g(1) - g(0) = f ( x o s t ) - f ( x o in s and 1, hence, by exchanging s and t , we get li(f”(xo)

+ s) +f(xo) is symmetric

< 6~(ilsll+ 11t11)’. < +r‘; but if we replace s and t

. t > . s - (f”(xO>+ s> . tll

Now this inequality holds for ~~s~~ < fr’, lltll by Is and I t , both sides are defined and multiplied by 1/212, hence the result is true for all s and t in E, in particular for llsll = /It11 = 1, which proves (by (5.7.7))that

llf”(xo> . ( 1 , s) -f”(x,> . (s, t>ll < 248 114 . lltll for all s and 1 ; as E is arbitrary, this ends the proof. In particular,

12 HIGHER DERIVATIVES

181

(8.12.3) Let A be an open set in R” (resp. C“); f a mapping f of A into a

Banach space F is tccdce differentiable at xo , then the partial derivatives D if are differentiable at xo, and Di Dj f ( x 0 ) = Dj Di f ( x 0 ) for 1 < i d n , 1 <,j
By induction on p , we now define a p times diflerentiable mapping f of an open subset A c E into F as a ( p - 1)-times differentiable mapping whose ( p - 1)th derivative D p - - ’ f is differentiable in A, and we call the derivative D ( D p - - ’ f )the pth deriuatiue off, which is written Dpf or f p ) . The element D P f ( x 0 )is identified to an element of the space 8 , ( E ; F) of the p-linear continuous mappings of E into F, and we write it (ti,

t2

9

. . . , t P ) DPf(x0) ( t l , . . ., t p ) . +

As in (8.12.1) we see that the mapping ti

+

DPf(xo) . (ti, t 2 , . . ., t p )

is the derivative, at x o , of the mapping

x -+ D ” - ’ f ( x ) . ( t , , . . . , t,). (8.12.2) generalizes to

(8.12.4) I f ’ f is p times differentiable in A, then the multilinear mapping D P f ( x )is symmetric f o r ench x E A.

This is proved by induction on p . Let t 3 , . . . , t , be fixed, and consider the mapping x -+ g ( x ) = D p - ’ f ( x ) . ( t , , . . . , t,); from the preceding remark it follows that the second derivative of g at x is

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DIFFERENTIAL CALCULUS

On the other hand, for any permutation the inductive hypothesis yields

0

.

of the set of indices {2,3, . . . p ] ,

D”-tf‘(x) . ( t u ( 2 ) , ta(3),. . . , tu(p))= D P - ‘ f ( x ). ( t 2

9

t3,

.. . >t p )

and taking the first derivative of both sides (where the t i are fixed), we obtain (8.12.4.2)

DPf(x) . ( t l , t c ( z ) ,. . . , f u ( p ) ) = DPf(,u) . ( t l , t 2 , . . . , tp).

Combining (8.12.4.1) and (8.12.4.2) we first see that DPf(x) . ( t i , t 2 , . . . , t p ) does not change when the index 1 is exchanged with any other index, and also when any two of the indices 3 2 are exchanged; but these transpositions generate any permutation of the indices I , 2, . . . ,p . Q.E.D. (8.12.5) Iff is m times differentiable and Dmf is ti times differentiable in A, then f is m n times differentiable in A, and Dmir’f= D”(D“f).

+

on

This is the definition when applying the definition.

ti =

1 , and is proved immediately by induction

11,

Suppose f = (fl, . . . ,f”,) is a continuous rnappitig of an open subset A into a product F, x . . . x F, of Banach spaces. In order that f be p times difJi.rentiable in A, it is necessary and siijicient that each fibe p times differentiable in A, and then Dpf = ( D p f , , . . . , D”J;,,).

(8.12.6) of E

This follows from (8.1.5) by induction on p . Let A be an open set in R” (resp. C“); i f a mapping f of A into a Banacli space F is p times dderentiable, then, f o r t , = ( t i j )( 1 < i < p , 1 < j < t i ) we have

(8.12.7)

D ” f ( x ) . ( t , , . . ., f p ) = (.it.

C....

ir,

Dj~Dj~..‘DjPf(X)~~,j,~z,j,.’.~p,j j,)

tlie sum being extended to all np distinct sequences (j,),,,,, 11, nl.

of integersf r o m

This is immediately proved by induction on p , using Section 8.1 0. The n p elements D j , D j , . D j p f ( x ) are called the partial deriuatiues of order p off at x ; any two which differ only by a permutation of the indices are equal by (8.12.4). We say that f is p times continuously differentiable in A if DPf exists and is continuous in A.

12 HIGHER DERIVATIVES

183

(8.12.8) Let A be an open subset of R” (resp. C“), f a continuous impping of A into a Bunuch space F; if the t i p partial dericatiws of f exist and are continuous in A, then f is p times continuously diflerentiable in A. For p = 1, this is (8.9.1) (extended to a product of n spaces); in general, we only have to use induction on p and the formula (8.12.7). We say that f i s indefiriitel~~ diflerentiahle in A if it is p times differentiable in A for any p ; all the derivatives D p f are then indefinitely differentiable in A.

Example (8.12.9) Any continuous bilinear tiiapping is indejnitely differentiable, nnd all its deriratiiw of order 2 3 are 0. From (8.1.4) it follows that the derivative of a bilinear continuous mapping at (s,y ) is (s, t ) -+[x . t ] + [s . y ] ; write g ( x , y ) E 9 ( E x F; G) that linear mapping; by assumption and (5.5.1), there exists c > 0 such that 11 [x . y]ii d c llxll . lly//in E x F; by definition ofthe norm in Y ( E x F; G) (5.7.1), we have llg(x-3Y)I/ Q C ( l I ~ l l + IlJ~ll)6 2c SUP(ll.~lI> IIJ~II)

hence g is a continuous linear mapping of E x F into 9 ( E x F; G), and therefore (x, y ) + [x . J!] is twice differentiable and its second derivative at (x, y ) is (by (8.1.3) and (8.12.1)) (($1,

tl)?(s2 t2)) - + b l 3

. f 2 1 + i s 2 . tll.

This is a mapping independent of ( x ,y ) , hence the result. (8.12.10) Let E, F, G be three Baiiacli spaces, A an open subset of E, B an open subset of F; iff is a p times continuously diflerentiable inupping of A into B, g a p times continuously clifferetitiable mapping of B into G , tlien 11 = g o f is a p times continuously chflc~rentiahleniappitig of A into G. For p = 1, the result follows from (8.2.1) and from the fact that u is a bilinear continuous mapping of 2 ( E ; F) x 9 ( F ; G) into (u, 11) + I ’ 9 ( E ; C) by (5.7.5). Use now induction on p ; as li’(x) = g ’ ( f ( x ) ).J”(x), and ,f and g’ are p - I times continuously differentiable, the induction hypothesis shows that g’ c , f ’ is p - 1 times continuously differentiable; from (8.12.6) and (8.12.9), it then follows that 11’ is p - 1 times continuously differentiable, hence Ii is p times continuously differentiable by (8.12.5).

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Example

Suppose there is a linear homeomorphism of a Banach space E into a Banach space F, and let &? c 9 ( E ; F) be the open set of these homeomorphisms i n Y ( E ; F) (8.3.2). Then the mapping u .+ u-' of &? onto 2- is indejitiitely differentiable. We prove by induction on p that u -+ u-' is p times differentiable, the property being true for p = 1 by (8.3.2). Given v and w in 9(F; E) = M, let f ( v , w) be the linear mapping t - u t IO of L = 9 ( E ; F) into M ; it is clear that f is bilinear (and maps M x M into 9 ( L ; M)) and (5.7.5) proves that Ilf(v, w)II 6 \lull * I\wII, hence f is continuous, and therefore indefinitely differentiable by (8.12.9). Now the first derivative of u -+ u-' is, by (8.3.2), the mapping u - + f ( u - l , u - ' ) ; by (8.12.6) and (8.12.10) it follows that if u-+u-' is p times differentiable, so is u -+f(u-', u - ' ) , and therefore, by (8.12.5), u -+ u-' is p 1 times differentiable. (8.12.11)

-+

+

Remark. When f is a mapping of an interval J c R into a real Banach space F, we have defined earlier (Section 8.4) the notion of derivative off at toE J with respect to J. By induction on p , we define the pth derivative o f f at to,with respect to J, as the derivative at to (with respect to J) of the ( p - 1)th derivative o f f (which is therefore supposed to exist in a neighborhood of to in J); it is an element of F, written again Dpf(to) orf(p)(
PROBLEMS

1. Letfbe an n times differentiable mapping of an interval I c R into a Banach space E. 1 Show that for any x E I such that - E I X

2.

(use induction on n). (a) Let p be the function defined on R by the conditions:

(

p(t)=exp -

1

iir;Ti)

for -. l < t < l

12 HIGHER DERIVATIVES

185

Show that the function p is indefinitely differentiable in R. (Use the relation lim x"e-x = 0 for any n > 0.) X'+

m

(b) In this problem, wc agree to extend any regulated functionfdefined in a compact interval [a, b] of R , to the whole of R, by giving it the value 0 for f < a and for f > b ;

we then write

I

+m

--m

f ( f )df for the integral

Jabf(f)

dt, which is also equal to Jcdf(t)dt

for c < a and d > b. For any such function f, let

j

r+m

f.(f) = nc

where l / c

=I+I

-m

f(s)p(n(t - s)) ds = nc

p ( f ) df ("regularization"

of

j

"+a,

-m

f by

f ( t - s)p(ns) ds

p ; we write p,(f) = ncp(nt) and

= f * pJ. Show that f, is indefinitely differentiable and vanishes in the complement of a compact interval (use (8.11.2); i f f is real and increasing (resp. strictly increasing, resp. convex) in [a, b], then J , is increasing (resp. strictly increasing, resp. convex) in [a l / n , b - l/n]. I f f (extended to R) i s p times continuously differentiable, then

fn

+

I:," I-

D P f ( f1 = nc

+m

= nc

(c) Show that for any n,

(DPf(s))p(n(f- s)) ds (Dpf(t- s))p(ns)ds.

J'_tIf(f) 5_' df =

mf(f) m

df.

(d) I f f (extended to R) is continuous (resp. p times continuously differentiable), then the sequence (f.) (resp. Dpfn) converges uniformly in R tof(resp. Dpf). (e) To what limit doesf(r,) ( f o E R) tend whenfis only supposed to be regulated in [a, h] (first consider the case in whichfis a step-function, then use (7.6.1)). (f) Show that for any regulated function f i n [a, b], "k

3. Let f be an n times differentiable real function defined in 1 - 1 , I[ and such that If(r)l < 1 in that interval. (a) Let mk(J) be the smallest value of / f ( k ) ( f ) l in an interval J contained in 1 - 1 , l [ . Show that, if J is decomposed into three consecutive intervals J,, J,, J 3 , and if J, has length p, then, for k < n,

(use the mean value theorem). Deduce from that inequality that if J has length A,

mk(J)<

2k(k+l)lZkk ~

A'

(use induction o n k ) . (b) Deduce from (a) that there exists a number a. depending only on n, such that if If'(0)l > a n , f ( " ) ( f= ) 0 has at least n - 1 distinct roots in 1- 1 , 1[. (Show by induction on k that there is a strictly increasing sequence xk, < xk. < . . . < x k , of points of 1- I ,I[ such that f(')(xr, i ) f ( k ) ( ~ k , +,) < 0 for I < i < k - 1; use Rolle's theorem.)

,

,

186

4.

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DIFFERENTIAL CALCULUS

Let E, F be two Banach spaces, A an open subset of E, f a n n times differentiable mapping of A into F. Let xo E A, hi E E ( I < i < n ) be such that xo f o r O < ( , < l , l < i < n . Wedefinebyinductiononk(1 < k < n )

E

A

+ hi) - f ( ~ o )

A'f(xo; h l ) =f(xo

Akf(x,; h i , . . . ,h,)

"

+ tihr

= A"'gr(xo

; h i , ...,h k - i )

with SkW =f ( x

+

-Ax).

hk)

(a) Show that

IlA"f(xo;hi, ..., h " ) l l ~llhill' I l h 2 I l ~ ~ ~ I IlD"f(z)/I lh~Il~~~ Z t P

where P is the set of points x o (b) Deduce from (a) that Ilhlf(xo ; hi,

< llhi I/

"

+ C tih i ,0 < ti< I . (Use induction on n.) i=i

...,h.1-

.

D"f(xo) (hi, . . ,hnN

llhz /I' ' . llhnll SUP IID"f(z)

- D"f(xo)ll.

Z E P

5.

Let f be a continuously differentiable mapping of an open subset A of R2 into a Banach space E. Suppose that in a neighborhood V of (a, h ) E A, the derivative D2(Dlf) exists and is continuous. (a) Let (x,y ) E V; show that for every E > 0, there exists 6 > 0 such that the relations /hi < 6 , lkl < 6 imply lIA2f(x, Y ; h, k ) - Dz Dif(x,

~ W l
(apply the mean value theorem of the function .dt)

f(x

+ t, Y +k)

-f

(x

+

f,

V ) - DZD i f ( x , y)tk

and use (8.6.2)). (b) Prove that D1(DZf) exists in V and is equal to Dz(Dlf) (use (a)). (c) Give an example of a function f satisfying the previous assumptions and for which DI(Dlf) and Dz(Dzf) do not exist anywhere (see Section 8.4, Problem I). 6. Let f the real function defined in R 2 by the conditions f(0,O) = 0, f ( x , y ) xy(x2 - y 2 ) / ( x 2 y 2 ) for (x, y ) # (0,O). Show that all four derivatives D1(Dlf), Dl(D,f), D,(D,f), D2(Dzf) exist everywhere in R2 but that DI(D2f) # D2(Dlf) at the point (0,O). 7. The notations are the same as in Problem 2 of Section 8.9. Let gn(t)= ?/(I n Iri),

+

:

+

and fn(f) =j:gn(u) du for every t E R. Show that the function f : (4")

--f

(fn(tn)) is

continuously differentiable in E, and that for each y = (7.) E E, the mapping . y is differentiable at x = 0, but that f' is not differentiable at that point (compare (8.1 2.1 )). 8. Let E, F, be two Banach spaces, A an open subset of E, @')(A) the vector space of p times continuously differentiable mappings of A into F, such that f and all its x +f'(x)

12 HIGHER DERIVATIVES

derivatives Dkf (1

187

< k < p ) be bounded in A. For any f~ L@’)(A),let llfll,

= SUP XE

(Ilf(x)lI

A

+ IIDf(x)lI + ... + IlD”f(~)ll);

show that llfll, is a norm on @P)(A) for which that space becomes a Banach space (use (8.6.3)). The mapping f-Df is a continuous linear mapping of C@)(A) into Bp-’)(A) (resp. in %’& F)(A) for p = 1). 9. Let E, F, G be three Banach spaces, L, M, N the Banach spaces @Y)(E), 9@)(F), 9g)(E), respectively. F o r f E L, g t M, let Q,(f, g) = g N. (a) Let (fo, go) E L x M. Show that if DPgo is uniformly continuous in F, the mapping Q, is continuous at ( f o , go) (use induction on p ) . If E, F, G are finite dimensional, 0 is continuous in L x M (use (3.16.5)). (b) Let Nt = 9g-,)E) for 1 < k < p with Yg)(E) = ZLm)(E).Show that, as a mapping of L X M into N1, CD is continuous at every point; in order that Q, (as a mapping of L x M into N,) be differentiable at ( f o , go), it is sufficient that Dpgobe uniformly continuous, and the derivative DQ,is the linear mapping ( ~ t0) ,

+((Dgo)

ofo)

’

+v

ofo.

(c) Let U, be the linear mappingg +. g o f of M into N ; show that U, is continuous. We may also consider U, as an element of Y ( M ; N,) for any k < p . Show that the mapping f - U, of L into P ( M ; N,) is continuous, and that the mapping f - U , of L into Z ( M ; NZ) is differentiable, the element DU, E Y ( L ;9 ( M ; N,)) being the bilinear mapping ( u , u ) ((Du) o f ) . 11. (d) Deduce from (b) and (c) that as a mapping of L x M into Nk,@ is k - 1 times differentiable. 10. Let f be a real valued twice differentiable function defined in an open subset A of a Banach space E. (a) Suppose that at a point xo t E there is a constant c > 0 such that Df(xo) = 0 and D*f(xo). ( I , t ) < -cl/tllZ for every t E E. Show that f reaches a strict relative maximum (Section 3.9, Problem 6 ) at the point x,,. If E is finite dimensional, the preceding condition can be replaced by the condition D2f(xo). ( t , t ) < 0 for any t # 0 in E (use the compactness of the sphere lltll= 1 in E). (b) Suppose A is an open ball, f i s majorized in A, and there exists a real number a such that the set F of points x E A for which f ( x ) 3 a is a non empty closed set in E (hence a complete subspace), and finally D2f(x) . ( 1 , I ) < --cl(t/12 for every x E A and every t E E. Show that under these conditions,freaches its maximum in A at a single point. (Let m = supf(x), which is a finite number; for every E > O such that a < m - E < rn, let F, c F be the set of x E A for which f ( x ) > m - E . Prove that the diameter of F, tends to 0 with E . To do this, first prove the following lemma: if h is a twice continuously differentiable real function in [0, 11 and h”(t) < b in [0, 11, then h(0) - 2 h ( $ ) h ( l ) < b/4. Apply the lemma to the function t +f(x t [ ) for x and x ( in A). 11. (a) Let f be a real valued function defined in an open interval I c R, and differentiable in I; let [a, b] c I , and supposef” exists in the open interval ] a ,b[, butf’ is not necessarily supposed to be continuous at a and b (cf. Section 8.7, Problem 6). Show that there exists c t ]a, h[ such that f’(b) - f ’ ( a ) = (b - a)f”(c) (use Problem 3 of Section 8.5). (b) What is the corresponding property for functions defined in I and with values in a Hilbert space (see Section 8.5, Problem 6)? --f

+

+

+

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13. DIFFERENTIAL OPERATORS

Let A be an open set in R” (resp. C“), F a real (resp. complex) Banach space; we denote by €‘,P)(A)the set of all p times continuously differentiable mappings of A into F. It is clear by (8.12.10) that &“) (A) is a real (resp. complex) vector space; and, more generally, (8.12.1 0) shows that &g)(A) (resp. &$“(A)) is a ring, and &!)(A) a module over that ring. For any system (al,. . . , a,,) = a of integers 2 0 with la1 =

1 aid p , let M a = Xdj’ X.,2

X:

i= 1

and define D” or DMaasthe mapping D:’ D.,2 * . DF of @)(A) into @‘-lal)(A).

A linear dflerential operator is a linear combination D

c aaDa where n

=

i=l

la1 < p and the aa are continuous scalar functions defined in A; if a, = 0 for la\> k and each a, is ( p - k ) times continuously differentiable, D maps bP)(A) linearly into &P-k)(A). (8.13.1) If the operator is identically 0 in A.

a,Da is identically 0, then each of the functions a, a

Write Df= 0 for f(x) = c * exp(A,(, + * * * + A,,(,,), where c # 0 is in F and the Ai are arbitrary constants; we obtain (by Section 8.8 and (8.4.1))

aa(x)Ma(Al,. . . , A,,)

identically in A, which is equivalent to

= 0;

for any

a

particular x E A, this implies aa(x) = 0 for each a, since the Ai are arbitrary. The coefficients a, of a linear differential operator are thus uniquely determined; the highest value of la1 such that a, # 0 is called the order of D. To each polynomial P =

1b,Ma

of degree


a

we can thus associate a linear operator D,

=

with constant coefficients

c b,Da

of order d p ; it is

= D,, + DP2,and it follows from (8.12.3) that if PIP, clear that DP,+P2 had a total degree d p , then DPIPz= D,,D,,. In particular, from (8.12.7) if follows that for fixed tij,the operator f + DA where a

Df(x) can be written

= D”f(x)

. ( t l ,..., t p )

13 DIFFERENTIAL OPERATORS

Let P(X,, . . . , X,) be a polynomial ofdegree

(8.13.2) (Leibniz's formula) < p , and suppose

p(x,

189

+ y,, . . . , x, + y,) = 1k Y k M;(Xi,

. . . , X,)Mi(Yi, . . , y,),

the M; and Mi being monomials. Let (x,y ) --+ [x . y ] be a bilinear continuous mapping of E x F into G . Then, for any mapping f E @)(A) and any mapping g E d'P)(A), [f . g ] belongs to dg)(A) and we have

It is enough to prove the formula when P is a monomial M ; using induction on the total degree of P, we can suppose P = X , M , hence D, = Di D M . We have by assumption DM[f'

gl

=

1 k

ykCDM;

f

'

DM,

gl

hence by (8.1.4) DP[f'

gl

=

1 k

yk(CDi

DM;

f

*

DM;

gl

+ ED,; f

' Di DM,

gl>

which we can write

1 h

y;CDNL

f

'

DN;i

gl

the summation being extended over all pairs of monomials ( N X X , , . * . , X,), N P , ,. . .,Y.)) such that either NI, = XiM; and Ni = M i for an index k , or NI, = M; and Ni = Y iM i for an index k ; there is exactly one such index k for each suitable index h, and we have y; = yk . The result is then obvious.

PROBLEMS

1. Let A be an open subset of R",E, F, G three Banach spaces, ( x , y ) [ x . y ] a continuous bilinear mapping of E x F into G . Show that the mapping (f, g ) [ f . g ] of QF)(A) x L&")(A) into LBg)(A) (Section 8.12, Problem 8) is continuous. 2. Let 1 be any compact interval in R, J an open neighborhood of 1. Show that there exists an indefinitely differentiable mappingfof R into [0, I], which is equal to 1 in I and to 0 in the complement of J (consider the functions g * pn (Section 8.12, Problem 2) where g is equal to 1 in a compact interval K such that I c K c -I, and to 0 in R - K). If u is a n indefinitely differentiable mapping of R into a Banach space E, show that there exists an indefinitely differentiable mapping u of R into E such that u ( t ) = u(f) in I, u ( r ) = 0 in R - J. --f

--f

Vlll

190

DIFFERENTIAL CALCULUS

14. TAYLOR’S FORMULA

Let I he ail open ititerrul iii R, j ; g two functions of dg)(I) arid (.Y, y ) + [x .4’] a coritinuous bilitiear mappiiig of E x F into G . Theii

(8.14.1)

&g)(l)respectiwly.

This is immediately verified by application of (8.1.4). (8.14.2) Let I be mi operi ititerval in R,f afirtictioti of Gp)(l);tlzeii, f o r any pair ofpoints a , 5 in I

Apply (8.14.1) to the bilinear mapping

(A,x) +Ax and to the

fuiictioii

5.

g(i) = ( 5 - [ ) p - l / ( p - l)!, and integrate both sides between a and

(8.14.3) Let E, F be tu’o Banaclr spaces, A at? open subset of E, f a p-times contiiiuously diflereiitiable mopping of A into F. Then, if rhe segrnetit ,joinirig x and x t is iti A, tile h a w

+

where t ( k )stands.for (t, t , . . ., t ) ( k titiies). IIi particular, ,for ererj, E > 0, there is r > 0 sirclz that for l(tll 6 r

I

f(x

+ t) -f(x)

1

--f’(x).

l!

t

1

- -f”(x) 2!

< E Iltll”.

1 t”’ - . . . - - f ‘ ” ‘ ( x )

‘

P!

*

t(P)

I

14 TAYLOR'S FORMULA

191

+

To obtain the first formula, apply (8.14.2) to the function g ( [ ) = f ( s [ t ) in the interval [0, I]; by (8.12.10), g is y times continuously differentiable, and it is immediately seen by induction on k that g("([) = , f ' k ' ( ~ tt) . t'k', using (8.4.1) and (8.1.3). To get the second formula, observe that by continuity of j'(p', r can be chosen such that lIf(P'(s i't) -f(p'(-x)ll < p ! & for 0 < i < 1 and lltll < r. Then the mean value theorem (8.7.7) yields

+

I/ s

+

(1 - l

+

y - 1

f("'(x it) cii - - p y x ) ( P - I>! P. and the conclusion follows from (5.5.7). 0

I/ <

&

PROBLEMS

1. The 17th Legendre polynomial is defined by 1 -D"((t' 2"n !

-

(a) Show that up to a positive factor, P, is the nth term in the sequence obtained by orthonornialization in the prehilbert space 'Kc(]), with I = [ - I , + I ] , from the sequence (1") (Section 6.6). (To prove that the scalar product of P,(t) and of the t"' with m < I? is 0, use (8.14.1)). (8.13.2)). (b) Show that P,,(l)= l,P,,-I)=(-l)"(use (c) Show that between three consecutive Legendre polynomials there is the following recursive relation 17Pn(l)- (2/7 -- l ) f p n - i ( f ) ( I 1 - l ) p n - Z ( f ) 0.

+

(Observe that if c. is chosen such that PJt) - c,,iP,-,(t) has degree < n - 1, it is orthogonal to the t k with k < 17 - 3, hence must be a linear combination of Pn-z and P , - , ; use also (b).) (d) Show that all the roots of P,, are real and simple and in 1- 1 , 1 [ (if P, changed sign at k < / I 1 points only in 1- I , 11, there would be a polynomial g ( f ) = ( t -. I , ) . . . (t - t k ) such that P,(t)y(t) 2 0 for -1 < t < 1 ; show that this leads to a contradiction with the fact that P,(t) is orthogonal to t h for h < 11). (e) Show that P, satisfies the differential equation -

(I 2.

- t')P,"(t)

-

2tP,(t)

+ n(a + l)P"(/)

=0

(show that D((I - t')PA(t)) is orthogonal to f k for k < 17). (a) Let f b e a real k times continuously differentiable function defined in an interval I C R of length a, and suppose that in I , l f ( k l ( f )2 i c > 0. Show, by induction on p . that for 0 -< p < k , there exists an interval I, c I of length a/4, such that in I, the inequality ~ f ' " - " ' ( t ) ~2 can/4" holds. (b) Letfbe a real k times continuously differentiable function defined in an interval I C R of length ( I , and, for 0 < p < k , let M, = sup If(-"'(t)l.Show that f E I

192

Vlll

DIFFERENTIAL CALCULUS

(c) The assumptions being those of (b), suppose that in addition k [-a/2, a / 2 ] . Show that for every t E I,

=2

and I =

(use Taylor's formula to express f ( a / 2 )-f(/)and f ( - a / 2 ) - f ( f ) ) . Deduce from that inequality that, if a 2 2(Mo/M2)'/2, then M I < 2(Mo M2)'12. Show that in this inequality the number 2 cannot be replaced by a smaller one. (Iff' is only supposed to have a derivative on the right!:, both sides of the inequality may become equal when f' is piecewise linear; use then Problem 2(d) of Section 8.12.) (d) L e t f b e a real twice continuously differentiable function defined in R , and such that Mo sup i f ( r ) i and M 2 = sup If"(t)l are finite. Show that M I = sup lf'(t)l is :

t e R

f E R

t E R

then finite and M I < (2Mo M2)'12 (use (c)). In that inequality, the number 2/2 cannot be replaced by a smaller one (same method as in (c)). (e) Show that iffis a red k times continuously differentiable function defined in R , and if Mo = sup I f ( / ) / and Mk = sup If("(t)l are finite, then, for 1 < p 6 k - I , the numbers M,

t E R

= sup

i t R

j f ( P ' ( / ) lare all finite, and one has

t e R

M,

<~P(~--P)M~-'P/~)MP/~.

(First show that M k - l is finite by using (b), then use induction on k > 2 and (d).) 3. Let E, F be two Banach spaces, A an open ball in E (or the whole space E). Show that in the space 9$')(A), the norm su~(llf(x)ll xs A

4.

+ IIDPf(x)Il)

is equivalent (Section 5.6) to the norm IlfII,, defined in Problem 8 of Section 8.12 (use the result of problem 2(c)). Let E be a Banach space, (c,) an arbitrary sequence of elements of E. (a) Let f be an indefinitely differentiable mapping of R into [0, I], equal to I in [-4, $1, and to 0 in the complement of [- 1, I ] (Section 8.13, Problem 2). Consider the series

Show that, for a suitable choice of the sequence of numbers t, > 0, that seriesconverges for every x 6 R, and for every integer m 2 I , the series of mth derivatives

+

is uniformly convergent in R (one can take t , = if /lc.// < 1, t. = l/211cnll if llc.ll > 1 ; use Leibniz's formula to majorize the terms of the series). Deduce that 11 is indefinitely differentiable in R and that, for every m >, 0, one has D"u(0) = c,, ("E. Borel's theorem "). (b) Prove in the same way that, given an arbitrary family (c,) of elements of E, where a = (aI,. . . , a,,)ranges through all systems of p integers a,> 0, there exists an indefinitely differentiable mapping f of R P into E such that D"f(0) = ca for every a. (c) Deduce from (a) that if g is an indefinitely differentiable mapping of a closed

14 TAYLOR'S FORMULA

193

interval I c R into E, and J an open interval containing I, there exists an indefinitely differentiable mappingfof R into E which coincides with g in I and with 0 in R - J. 5. Let f b e a mapping of an interval 1 c R into a Banach space E, and supposefis n times differentiable at a point a E I. Show that

(use induction on n and (8.5.1) with v([) = ([ a)"-'). 6 . Let I c R be an interval containing 0, f an n - 1 times differentiable mapping of I into a Banach space E. Write -

which definesf, in I - {O}. (a) Show that iffis n p times differentiable at t = 0, f.can becontinuouslyextended to I and becomes a function which is n p - 1 times differentiable at all points t # 0 in a neighborhood V of 0 in I, and p times differentiable at t = 0; furthermore f:(O) = k! f'"''(0) for 0 < k < p , and lim fAP+'"'(t)tk = 0 f or 1 G k ~ n - l . r-0, t t o . r E v (n k ) ! (Express the derivatives off. with the help of the Taylor developments (Problem 5 ) of the derivatives off, and use Problem 2 of Section 8.6.) (b) Conversely, let g be an n p - 1 times differentiable mapping of I - {0} into E, such that lim g ' p f k ' ( f ) fexists k for 0 < k < n - 1. Show that the function g can

+

+

+

+

r-+O.tto,re~

be extended to a p - I times differentiable mapping of I into E, and that the function g(t)t" is n p - I times differentiable in 1; if furthermore g'p)(0)exists, then g(t)t" is n + p times differentiable at 0. (c) Suppose I = 1- 1, I[, and supposefis euen in I , i.e.f(-t) = f ( t ) . Show, using (a) and (b), that i f f is 2n times differentiable in I, there exists an n times differentiable mapping h of I into E such that f ( t ) = h(r2). 7. (a) Let f b e an indefinitely differentiable mapping of R" into a Banach space E. Show that

+

. .,X") =fa. . . , 0) + xlfI(x1, .. . ,X") + x2 f2(x* ...,x.) + . . . + x. f.(xJ where fk is indefinitely differentiable in R"-k+l (1 < k < n). Write f ( x l , .. . , x,) = (f(xl,.. . , x.) - f ( O , x 2 ,. . . , x.)) +f(O, x 2 ,. . ., x.) and apply (8.14.2) to the first f(x1,.

1

summand, considered as a function of x l ;with a suitable value ofp (depending on k ) , this will prove that ( f ( x l , ..., x,)-f(O, x 2 , . .., x,))/xl is k times differentiable at (0,. . . , O ) ; finally, use induction on n.) (b) Deduce from (a) that for any p > 0,

where all the fa are indefinitely differentiable and f o ( x )= f ( O , . . . , 0). (c) Let f be an indefinitely differentiable mapping from R" into R; suppose that f(0) = 0, Di f ( 0 )= 0 for 1 < i < n, and that the quadratic form ([I,

. . .*t n ) E Dt Dj f(0)ti i.j +

t j

194

Vlll

DIFFERENTIAL CALCULUS

is nondegenerate. Show that, by using a linear transformation in R" one can assume that DlDjf(0) = 0 for i # j and a1 = D:f(O) # 0 for 1 < i < n. Prove that there exists a neighborhood U of 0 and n indefinitely differentiable functions g , defined in U, such that in U

and gi(0) = 1 for 1 < i < n. (Use again (a) applied to each of the functions fl defined in (a), and then apply the usual method of reduction of a quadratic form to a form having a diagonal matrix.) 8. (a) Let S be a metric space, A , B two nonempty subsets of S, M a vector subspace of the space V,(S) of real continuous functions in S, N a vector subspace of M, u +L(u) a linear mapping of M into the space RAof all mappings of A into R.We suppose that: ( I ) there exists a function u0 E N such that L(uo)is the constant 1 on A ; ( 2 )if u E Nand there is a t E B such that u(t) = 0, then there is x E A such that (L(u))(x)= 0. Let u E M such that L(u) = 0; show that for any function u E M such that u - u E N, and any t E B, there exists 6 E A (depending on t) such that u(r) = u ( t ) uo(f)(L(u))(@ (Observe that u o ( t ) # 0, and therefore there is a constant c (depending on t) such that u ( t ) - v ( t ) - cuo(t) = 0). (b) Suppose S is compact, A is connected and dense in S, and all functions u E N vanish on S - B. Suppose that L(u) is continuous in A for every u E M, and that if a function u E N is such that (L(u))(t)> 0 for any t E A, then u has no strict maximum on B. Show that in such a case condition ( 2 ) of (a) is also verified. 9. (a) Let f be an n times differentiable real function defined in an interval 1; let XI < x 2 < ... < xp be points of I, n i (1 < i < p ) integers > O such that nl n2 . . . np = n. Suppose that at each of the points x l , fCX'(x,)= 0 for 0 < k < nl - 1. Show that there is a point 4 in the interval ] x l , x p [ such that f("-')([) = 0 (apply Rolle's theorem iteratively). (b) Let g be an n times differentiable real function defined in I, and let P be the real ) P(k)(x,)for 0 < k < n, - 1 , 1 < i < p. polynomial of degree n - 1 such that g ( k ) ( x i= Show that for any x E I, there exists 6 in the interior of the smallest interval containing x and the xi (1 < i < p), such that

+

+ + +

(Use Problem 8(a), or give a direct proof, using (a) in both cases.) 10. Let g be a real odd function, defined and 5 times differentiable in a symmetric neighborhood I of 0 in R. Show that, for each x E I s(x)

X =3 (g'(x)

+ 2g'(O)) - 180 g y . 9 XS

where [ is a number belonging to the open interval of extremities 0 and x . Deduce from that result that, iffis a real function, defined and 5 times differentiable in [a, b ] , then

with a < 4 < b (" Simpson's formula").

14 TAYLOR'S FORMULA

195

11. Let I = [a, b] be a compact interval, and let Mo be the vector space of real continuous functions defined in I and such that, for any t E ]a,6[, the limit

.

exists in R.All real functions which are twice differentiable in I belong to Mo (a) Let M be the vector subspace of Mo consisting of functions f for which L (f)is continuous in ]a, b[. Show that any function of f~ M is twice differentiable in ]a, b[ and that L(f)=f". (Use Problem 8(a) and 8(b), taking S = I, A = B = ]a,b [ , and for N the subspace of M consisting of functions f for which f ( a ) = f ( b ) = 0.) (b) Show that the function f ( t ) = t cos(l/t) belongs to M o , although it is not differentiable at t = 0. 12. What are the properties of functions with values in a Hilbert space which correspond to the properties of real functions discussed in Problems 9(b), 10, and 1 1 ? (Cf. Section 8.5, Problem 6.) 13. Let f be an indefinitely differentiable mapping of a compact interval I = [a,b] c R (with a 3 0) into a Banach space. (a) Show that, for any two integers p , 4 such that 0 < p < q,

(For p < n < q, express f ( " ) ( a )using Taylor's formula at the point b.) (b) Under the same conditions, show that

(apply Taylor's formula tof'p) and use Problem 4(c) of Section 8.11). (c) Suppose f is indefinitely differentiable in the interval [a, +a[where a 3 0, and that for every integer n 2 0, there is a finite number M. such that Ilf'"'(x)Ilxn/n! < M. for every x 3 a . Show that for 6 > a and n < q, f ( " ) ( b )= -

s,'

(b - x ) l I - n - l f('J)(x)

(4-n-

I)!

dx

and for a < x < b

where the series and the integral are convergent (use Taylor's formula). Conclude that one has

and

(d) Show that, under the assumptions of (c), the function

(whose values are finite and 30,or f a ) is increasing in [a, +m[.

196

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DIFFERENTIAL CALCULUS

(e) Supposefis indefinitely differentiable in [a, +co[ and that lim f(")(x) = 0 x++m

for every n > 0.

Prove that the sequence of numbers (finite and 2 0 or equal to +a)

J.

=jotIlf(")(x)II

x"- 1

dx

(n > 1)

is increasing. (When these numbers are all finite, write f(")(x) =

-jx+mf("+l)(t) dt

and use Problem 4(c) of Section 8.1 1.) (f) Suppose f is indefinitely differentiable in [a, +to [ and that the integrals

are finite. Show that, for x 3 a,

14. Let f b e a n indefinitely differentiable mapping of the interval [a, +a[, where a 3 0,

into a Banach space. (a) Show that, for every p > 0,

where both sides may be equal to equal to both limits (w)

lim

+ co. Show that if both sides are finite, they are also

C llf(")(x)ll 2 m

x++m n = p

X'

and

lim n-rm

x"jCm llf(")(x)ll -dX (n - l)! 1

(I

(Use Problem 13). (b) I f f satisfies the assumptions of Problem 13(c), then the limits (**) always exist and are equal to both sides of (*).

CHAPTER I X

ANALYTIC FUNCTIONS

In this chapter, we have tried to emphasize the most general facts pertaining to the theory of analytic functions, and in particular to state as many results as possible for analytic functions of any number of variables; until Section 9.13, the theorems which concern only functions of one variable are inserted in a context in which they appear as technical intermediates to the general statements; it is only in Sections 9.14 to 9.17, and in many problems in this chapter and the next one, that we really deal with properties special to the one variable case. Furthermore, we have discussed simultaneously the case of analytic functions of real variables and of analytic functions of complex variables as long as it can be done (i.e. until Section 9.5). Finally, we have kept throughout our general principle of dealing from the start with vector valued functions; as usual, this does not require any change in the proofs, and the reader will see in Chapter XI how useful the consideration of such functions can be. Of course, one can only expect to find here the most elementary part of the very extensive theory of analytic functions. The definition is given by the local existence of power series representing the function, and it is by the technique of power series that the differential properties of analytic functions are obtained (9.3.5) (the usual definition of analytic functions by the existence of continuous derivatives only applies, of course, to functions of complex variables, and therefore that characterization is postponed until Section 9.1 0). The fundamental results about power series are Abel’s lemma (9.1.2) -from which is derived the vital possibility of substituting power series into power series (9.2.2)-and the principle of isolated zeros (9.1.5), whose most important consequence is the principle of analytic continuation (9.4.2), which expresses the “solidarity” between the values of an analytic function at different points of the domain where it is defined. From that point on, we have to assume that the variables are complex; 197

198

IX ANALYTIC FUNCTIONS

with the exception of the principle of maximum (9.5.9), all additional properties of analytic functions of complex variables derive from a single new idea, that of “ complex integration,” and from its fundamental features, Cauchy’s theorem (9.6.3), Cauchy’s formula (9.9.1), and its generalization, the theorem of residues (9.16.1).The form of Cauchy’s theorem which we give here is not the best possible, for it expresses the integral along a circuit as an invariant of the homotopy class of that circuit, whereas in fact it is an invariant of its homology class. In most applications, however, this has no inconvenience whatsoever, and in contrast to the fact that the proof of the weak form of Cauchy’s theorem needs almost no topological preparation, the proof of the complete theorem would have required some developments of algebraic topology, which we feel are above the level of the present volume. The interested reader will find the complete Cauchy theorem, together with all the necessary prerequisites, in Ahlfors [l], Cartan [8], and Springer [17]; we shall come back to that question in Chapter XXLV. Instead of using more results from algebraic topology in order to obtain such refinements, we have thought it might interest some readers to see how, by the very simple device introduced by S. Eilenberg, it is possible to obtain quite deep information on the topology of the real plane (including the Jordan curve theorem), using merely the most elementary facts about complex integration; this is the purpose of the Appendix (which, by the way, is not used anywhere in the later chapters and may therefore be bypassed without any inconvenience). As we have announced in Chapter I , the reader will find no mention in this chapter of the so-called “ multiple-valued ” or multiform ” functions. It is of course a great nuisance that one cannot define in the field C a genuine continuous function Jz which would satisfy the relation (Jz)’ = z ; but the solution to this difficulty is certainly not to be sought in a deliberate perversion of the general concept of mapping, by which one suddenly decrees that there is after all such a “function,” with, however, the uncommon feature that for each z # 0 it has titto distinct “values.” The penalty for this indecent and silly behavior is immediate: it is impossible to perform even the simplest algebraic operations with any reasonable confidence ; for instance, the relation 2Jz = Jz Jz is certainly riot true, for if we follow the “definition” of J z , we are compelled to attribute for z # 0, ticso distinct values to the left-hand side, and three distinct values to the right-hand side! Fortunately, there is a solution to the difficulty, which has nothing to d o with such nonsense; it was discovered more than 100 years ago by Riemann, and consists in restoring the uniqueness of the value of \/. by “doubling,” so to speak, the domain of the variable z, so that the two values of 2 / z corre“

+

1

POWER SERIES

199

spond to tit'o different points instead of a single z ; a stroke of genius if ever there was one, and which is at the origin of the great theory of Riemann surfaces, and of their modern generalizations, the complex manifolds which we shall define in Chapter XVI. The student who wishes to get acquainted with these beautiful and active theories should read H. Weyl's classic [19] and the modern presentation by Springer [17] of Riemann surfaces, and H. Cartan's seminar [7] and the recent book of A. Weil [IS] on complex manifolds.

1. POWER SERIES

In what follows K will denote either the real field R or the complex field C ; its elements will be called scalars. In the vector space K" over K, an open (resp. closed) polydisk is a product of p open (resp. closed) balls; in other words it is a set P defined by conditions of the form Izi- ail < ri (resp. Izi - ail < ri), 1 < i < p , on the point z = (zl, . . . , z,), with ri > 0 for every index; a = ( a l , . . . , a,) is the center or P, r l , . . . , r, its radii (a ball is thus a polydisk having all its radii equal). (9.1.1) Let P, Q be trr'o open polydisks in KP siich that P n Q # @; f o r any tn'o points x, y in P n Q , the segment (Section 8.5) joining x and y is contained in P n Q ; in particular P n Q is connected.

+

Indeed, if Ixi - ail < r i , lyi - ail < r i , then ltxi (1 - t ) y i - ail < tlxi - ail (1 - t)lyi - ail < r for 0 < t < 1 ; the last statement follows from the fact that a segment is connected (by (3.19.1) and (3.19.7)) and from (3.19.3).

+

,

We introduce the following notation: for any element v = ( n l , . . . , J?,) in NP ( n , integers 3 0 ) and any vector z = (zl,. . . , z,) E ISP, we write * + n p . If E is a Banach space zv = z1'z;'. . . z i p and IvI = n, + n2 + (over K), ( c , ) , , ~ ~a family of elements of E having NP as set of indices, we say that the family(c,z"), E N P of elements of E is a power series in p variables z i (1 < i < p ) , icsith coeflcients c, . E K P be such that b , # 0 f o r 1 < i Q p , and that the family ( c v b v ) be bounded in E. Then f o r any system of radii (ri) such that 0 < r < lbil f o r I < i < p , the portxer series (c,zv) is normally summable (7.1) in the closedpolydisk ofcenter 0 and radii ri ("Abel's lemma").

(9.1.2) Let b = ( b L ,. . . , 6,)

200

IX ANALYTIC FUNCTIONS

For if (/c,b’l( < A for any v EN!’, it follows from the definition of the norm in KP that if (zil < ri < [hi( (1 < i < p ) , we have ( ( c v z v< ( JAq’, with q = ( q l , , . . , q,), qi = ri/Jbil< 1. It follows from (5.5.3) that the family ( q v ) v p N of p positive numbers is absolutely summable, hence the result by (5.3.1). (9.1.3) Uiider the assumptioris of (9.1.2), the sum of the p o u w series (c, z‘) is continuous in the opeii polydisk of center 0 and radii Ibil.

As every point of that polydisk is interior to a closed polydisk of radii ri < / b i / ,the result follows from (7.2.1). Let q be any integer such that 1 < q < p ; for any v = ( n l , . . . , n,), write v‘ = (ti1, . . . , n,), V “ = (nq+,,. . . , n,); consider KP as identified to the product K 4 x KP-,, and for z = (zl . . . , z,) E KP, write z’ =.(z,, . . . , z,), Z” = (z,,,, . . . , z,). With these notations: (9.1 -4) Suppose the pouter series (c,z’) is absolutely summable in the polydisk P of radii ri aiid center 0 iii KP. Then, f o r any v” E NP-, the series (c(,.,, , ~ ~ ~ z ’ ’ ’ ) is absohrtely suinmable in the polydisk P’, projection of P on K4;let g J z ’ ) be its sum. Then, f o r any z’ E P’, the power series (g,.,(z‘)z“’”) is absolutely summable in the polydisk P”, projection of P 011 Kp-q, and its sum is equal to the sum of the series (c’z’). As z’ = Z”’Z’””, the fact that each of the series (q,,,,, , ~ ~ ~ Z ‘ “ Z ” ~ ” (v” ) fixed) is absolutely summable, and that gV,,(z‘)z”‘”= cvzv,follows from (5.3.5)

c

c

V”

V

and from the associativity theorem (5.3.6) for absolutely summable families. If we take Z” E P” such that zi # 0 for q 1 < i < p , the absolute summability of (C(,., ‘“)Z”’) follows.

+

(“ Principle of isolated zeros ”) Suppose (cnzn) is a power series (9.1.5) in one variable which converges in an open ball P of radius r, aiid

let f ( z )

=

2 c,zn.

Then, unless all the c, are 0 , there is r‘ < r such

n=O

that f o r 0 < ( z ( < r ’ , f ( z ) # 0.

Suppose h is the smallest integer such that cl1# 0; then we can write f( z ) = zh (c,, + c,,+,z + * * . + e l l + m ~+m* . * ) and the series (ch+,,,zm)converges in P; i f g ( z ) = c , + c , + , z + ~ * ~ + c , , + , , , z m +. . . , g iscontinuous in P by (9.1.3) and as g(0) = c,, # 0, there is r‘ > 0 such rhat g(z) # 0 for J z J< r’; hence the result.

2 SUBSTITUTION OF POWER SERIES IN A POWER SERIES

201

(9.1.6) Suppose two p o s e r series (a,zv) and (b,z") are absolutely summable and have the same sum in a polydisk P; then a, = b, f o r every v E NP. Use induction on p ; for p = 1, the result follows at once from (9.1.5). Taking the difference of the two power series, we can assume b, = 0 for m

I

every v ; applying (9.1.4) with q = p - 1, we have n=O

gn(z')z; = 0, hence

gn(z') = 0 for every n and every z' in the projection P' of P on KP-'; the induction hypothesis applied to each gn yields then a, = 0 for every v.

PROBLEMS

..

1. Let (cy z") be a power series in p variables zl (1 < i < p ) ; let a = (al, . ,a,) E Kp. In order that a real number Y > 0 be such that, for any r E K such that If) < Y, the series (c,(fa,)"' . . . (fa,)",) be absolutely summable, it is necessary and sufficient that

for all but a finite number of indices v = (nl, . . . , n,) (apply (9.1.2).) In particular, for p = I , there is a largest number R 2 0 (the "convergence radius," which may be .c) such that the series (c.z") is convergent for 121 < R, and that number is given by I /R = lim (s~p(IIc,+~/l'/("+~))), which is also written lim . supllc.ll'~"

+

n-m

k30

"-+W

(cf. Section 12.7). When in particular lim l/c,l/'/" exists, it is equal to l/R. "-m

2. Give examples of power series in one complex variable, having a radius of convergence R = 1 (Problem 1) and such that: ( I ) the series is normally convergent for 121 = R ; ( 2 ) the series is convergent for some z such that IzI = R , but not for other points of that circle; (3) the series is not convergent at any point of Iz/ = R. 3. Give an example of a power series in two variables, which is absolutely summable at

two points ( a l , a2), (bl, b2), but not at the point

"1.

bl, 2 +

(Replace z by

in a power series in one variable.) Let (c,,~"), ( d , , ~ "be ) two power series in one variable with scalar coefficients; if their radii of convergence (Problem 1) are R and R', and neither R nor R' is 0, then the radius of convergence R" of the power series (c,d,z") is at least RR' (takenequal to cu if R or R' is co).Give an example in which R" > RR'.

z1z2

4.

+

+

2. SUBSTITUTION O F P O W E R SERIES I N A P O W E R SERIES

Let Q be a polydisk of center 0 in Kq, and suppose the p power series in q variables ( b r ' u p ) with scalar coefficients are absolutely summable in Q (with p = ( m l , . . . , mq), u = (ul,. . . , uq), up = u y l . . . u,"~). We write

202

IX ANALYTIC FUNCTIONS

gk(u)=

bF’u”, Gk(u)=

c IbF)

[ u p .On the other hand, let (a,zv) be

a power

B

P

series in p variables with coefficients in E, which is absolutely summable in a polydisk P of KP, of center 0 and radii rk (1 < k < p ) . If, in a monomial zv = z;’ . . . z i p , we replace “formally” each z k by the power series gk(u), we are led to take the formal “product” of n, + n2 + + nP series, i.e. to pick a term in each of the n, + ... + nP factors, to take their product and then to “ s u m ” all terms thus obtained. We are thus led to consider, for each v = (nl, n 2 , . . . , nP) the set A, of all finite families ( p k j ) = p where p k j E Nq, k ranges from 1 to p , and for each k , .j ranges from 1 to n,; t o such a p we associate the element tP(U)

= a,

nn P

nk

bfpkJ.

k = l j=1

With these notations:

Suppose sl, . . . , sq are q numbers > O satisfying the conditions Gk(sl,. . . , sq) < rk for 1 < k < p . Then, f o r each u in the open polydisk S c Kq of center 0 and radii si (1 < i < q), the family (t,(u)) (where p ranges through the denumerable set of indices A = A,) is absolutely summable,

(9.2.1)

u

and i f f ( z ) =

VENP

1avzv,its sum is equal t o f ( g , ( u ) ,g2(u),. . . ,g,(u)). V

In other words, under the conditions G,(s,, . . . , sq) < rk (1 6 k < p ) , “substitution” of the series gk(u) for zk ( 1 < k < p ) in the series f yields an absolutely summable family, even before all the terms t,(u) having the same degrees in u l , . . . , uq have been gathered together. To prove (9.2.1), we need only prove that the family (t,,(u)) is absolutely summable; that its sum is f ( g , ( u ) , . . . , g,(u)) follows by application of the associativity theorem (5.3.6) to the subsets A, of A, and by using (5.5.3), which shows that t,(u) is equal to a,(gl(u))”* . . . (gP(u))“P.To prove the PEA,

family (t,(u)) ( p E A) is absolutely summable, we apply (5.3.4). For any finite subset B of A, we have, by (5.3.5) and (5.5.3)

and by assumption, the right-hand side of that inequality is the element of index v of an absolutely summable family; hence the result. Write t,(u) p k j = (mkj,,.

= cPul,

with ,A

= (,Al,

. . , mkjq)). From (9.2.1)

. , . , ,Aq), Izi

1 c mkji (if P

=

k=l

nk

j=1

we have

and (5.3.5) it follows (taking all the ui

3 ANALYTIC FUNCTIONS 203

to be ZO,u E S), that for each A, the family of the c p , where p ranges over all elements of A which correspond to the same 2, is absolutely summable in E; if d , is its sum, we see, by the associativity theorem (5.3.6),that

(9.2.1.1)

f(g,(u), ' .

. ?

g,(u)) =

1d i ui i

the series on the right-hand side being absolutely summable in the polydisk S. By definition, that power series is the ponier series obtained by substituting g k ( U ) to z k ,for 1 < k < p , in the power series ( a , 2').

(9.2.2) I f the point (gl(0),. . . ,gp(0))of KP belongs fo P, then there exists in Kq an open polydisk S such that, for u E S, the series gk(U) may be substituted to zk (1 < k < p ) in the poii'er series (a, z"). Observe that by definition, Gk(0)= Igk(0)l for 1 < k 0 (1 < i < q ) such that G,(s,, . . . , sq) < rk for 1 < k < p follows at once from the assumption.

3. A N A L Y T I C F U N C T I O N S

Let D be an open subset of KP. We say that a mapping f of D into a Banach space E over K is analytic if, for every point a E D, there is an open polydisk P c D of center a, such that in P, f ( z ) is equal to the sum of an absolutely summable power series in the p variables zk - a, (1 < k < p ) (that series being necessarily unique by (9.1.6)). Suppose K = C, let b be a point of D, and let B be the inverse image of D by the mapping x + b + x of RPinto Cp. Then it follows at once from the definitions that x +f ( b + x) is analytic in the open subset B of RP.

(9.3.1) Let (a,z") be an absolutely summable powjer series in an open polydisk P c KP. Then f ( z ) = a,zv is analytic in P; more precisely, if ri (1 < i d p ) V

are the radii of P, for any point h = (bi)E P, f ( z ) is equal to the sum of an absolutely summable poiver series in the zk - h k in the open polydisk of center b and of radii ri - lbil ( I < i < p ) . This follows at once from (9.2.1)applied to the case q = p , gk(u)= b, + z i k ; we have then G,.(u)= Ibkl + u k , and the conditions Gk(s,,. . . , sp) < rk (1 < k < p ) boil down to s, < rk - lbkl (1 d k
204

IX ANALYTIC FUNCTIONS

An entire function of p variables is a mapping f of KP into E which is equal to the sum of a power series which is absolutely summable in the whole space KP (cf. (9.9.6)). For each b E KP,f ( z ) is then equal to the sum of a power series in the zk - b,, which is absolutely summable in the whole space KP,by (9.3.1).

KP,B an open subset of K4, gk (1 < k < p ) p scalar functions defined and analytic in B, and suppose the image of B by (g,, . . .,g p ) is contained in A. Then, for any analytic mapping f of A into E, f ( g l , . . . ,9,) is analytic in B. (9.3.2) Let A be an open subset of

This follows at once from the definition and from (9.2.2). In particular, if f is analytic in A c KP, then, for any system ( a q + l , .. ., a,) of p - q scalars, (zl, .. . , zq) -,f( z l , . . . , z 4 , a4+1, .. . , a,) is analytic in the open set A(a,+,, ..., a,) (3.20.12) in K4. (9.3.3) In order that a mapping f = (fi,. . . ,&) of A c KP into K4 be analytic in A, it is necessary and suficient that each of the scalar functions fi (1 < i < q)

be analytic in A.

Obvious from the definition.

+ iyk f o r 1 < k < p , x, and Yk being real. Iff is analytic in A c Cp, then (x,, y l , . . . , x, , y,) +f (xl + iyl , . . . , x,+ iy,) is analytic in A, considered as an open set in RZp.

(9.3.4) Let zk = x k

Indeed, that function is analytic in the open subset B c C2P, inverse image of A by the mapping (ul, vl, . . . , up,up) + (ul + iv,, . . . , up + iv,) of C2" into C p , by (9.3.2). Hence it is analytic in A = B n R24 when A is considered as a subset of R2,.

(9.3.5) Let (c,,,,,~ . . . n p ~ ; ' . .. z?) be a power series which is absolutely summable in an open polydisk P of center 0, and let f ( z ) be its sum. Then the power

series (nkc,,,,, .. . ,,,z;' . . . zF-' . . . z?) is absolutely summable in P and its sum is the partial derivative D, f (= aflaz,).

3 ANALYTIC FUNCTIONS 205

For any z E P, we can in the series substitute zi to itself for i # k and

zk

+ uk to zk ,and we thus obtain a power series inp + 1 variables zl, . ..,z,,

+

uk,

which, by (9.2.1), is absolutely summable for (zil < r,(i # k)and l z k l IukI < r k (if r l , . . ., r, are the radii of P). By (9.1.4) we can therefore write f (z,, . .., zk uk, . . . , z,) =f ( z ) ukf,(z) -. uIf,(z) * * . , where each f, is a power series absolutely summable in P, and the right-hand side, for each z E P, is a power series in uk which is absolutely summable in some open ball B of center 0 (depending on z). Moreover it follows from the binomial theorem that

+

+

+

jl(z) =

C nk c,,

,

. , n p z;'

+

+

. . . z 2 - ' .. . 27

V

and as ( f ( Z 1 , ..., Z k + U k ,..., Z p ) - f ( Z ) ) / U k = f i ( Z ) + *" +ui-x(z)+ '*' is an absolutely summable power series (in uk) in B (for fixed z ) by (9.1.4), we deduce from (9.1.3) that f i ( z ) = Dkf ( z ) for any z E P. From that result and from (9.13) we deduce the values of the c, in terms of the derivatives off, namely (9.3.5.1)

V ! C,

where Dv = DY .. . DF and v! on IvI = n, n,.

+

+

= D"f(0)

= n, ! n2 ! . . . n,!

;this is immediate by induction

(9.3.6) An analytic function in an open set A c K Pis indefinitely differentiable and all its derivatives are analytic in A. This is an obvious consequence of (9.3.5) and (8.12.8). For p = 1, we have a "converse" t o (9.3.5): (9.3.7) Let (c,, 2") be a power series convergent in the baN P: IzI < r in K,and Zetf(z) =

2 cnz" in P. Then the power series ( ( l / ( n + I))c,,z"+') is convergent m

n=O

in P and its sum is a primitive o f f .

Due to (9.3.5) we have only to check the convergence of the series 1 c,,z"+'), which follows at once from the inequality

(z

and from (9.1.2).

206

IX ANALYTIC FUNCTIONS

PROBLEMS

1. Let (a.z"), (b,z") be two power series in one variable, the b, being real and >O; suppose lim a,/b, = s. n- m

(a) Suppose the series (b.2") is convergent for / z /< 1, but not for z = 1 (which means k

that if ck = C b,, lim ck = n=o

k-m

+ m). Show that the series (a.z") is absolutely convergent

for Iz/ < I , and that, if I = [0, I[,

(Observe that, for any given k ,

lim 2-1,

IEI

( cb,z")

=

+a).

nak

(b) Suppose the series (b,z") is convergent for every z. Show that the series (ant")is absolutely convergent for every z, and that if J is the interval [0, +a[in R, then lim ( F a n z n ) / ( g $ z n ) = s . z + + m . z ~ J n=O

(Same method.)

c c m

(c) Show that if the series (a,) is convergent and

a,

= s,

then the series (a.z") is

"=O

absolutely convergent for /zI < 1, and that

lim 2-1.

*El

m

a,, z" = s. (Apply (a) with b, = 1

"=a

for every n; this is "Abel's theorem ".) (d) The power series ((- 1)"~") has a radius of convergence 1, and its sum l / ( l z ) tends to a limit when z tends to 1 in I, but the series ((-1)") is not convergent (see Problem 2). 2. Let (a.2") be a power series in one variable having a radius of convergence equal to 1 ; letf(z) be its sum, and suppose thatf(1-) exists. If in addition lim nu. = 0, show that

+

n-m

the series (a,)is convergent and has a sum equal to f(1->. ("Tauber's theorem": observe that if 1na.l < E for n 1 k, then, for any N > k , and 0 < x < 1

and

3. Let (a,z")be a power series in one variable having a radius of convergence r > 0, and I) and 191 < r. let (b,) be a sequence of scalars # O such that q = lim ( b n / b n + exists n-ra

Show that, if C.

= sob.

+

lim (c,/b.) exists and is equal to f ( q ) . n- m

bn-i

+ ... + a.60

4 THE PRINCIPLE OF ANALYTIC CONTINUATION

207

4. Let (pnz"),(q.z") be two power series with complex coefficients, and radius of convergence #O, and let f(z) = C p n z " ,g(z) qnznin a neighborhood U of 0 where both

=c "

0

series are absolutely convergent. Suppose qo = g(0) # 0 ;then there is a power series c,z" which is absolutely convergent in a neighborhood V c U of 0 and has a sum equal to f(z)/g(z) in V (remark that the series (z") is convergent for IzI < 1 , and use (9.2.2)). If all the qn are >O, the sequence (q.+&.) is increasing, the pn are real and such that the sequence (p,/qn) is decreasing (resp. decreasing), show that c, 2 0 (resp. c, < 0) for every n >, 1 . (Write the difference

P"

Pn- 1

4.

4"-1

as an expression in the qk and c k , and use induction on n.) Deduce from that result that all the derivatives of x/log(l - x ) are
/Ip

z" dx dy = 0

for any integer n 3 1.

(Use the fact that Lebesgue measure on R2 is invariant by rotation (cf. (14.3.9)).) Deduce from that relation that for every function f which is analytic in an open set A c C containing P, one has

Show similarly that

Jlp

z " P dx dy = 0

if rn # n

4. T H E PRINCIPLE OF ANALYTIC C O N T I N U A T I O N

(9.4.1) In KP, let P, Q be two open polydisks of centers a, b, such that P nQ # Let (cnl .., n p ( ~ l- a,)"l . . . (xp- a,,)"p) be a power series in the x i- a i , absolutely summable in P, and let f ( x ) be its sum. Let (dnl... n p ( ~ l - bJ' . . . ( x p - bp)"p)be apower series in the x i - b i , absolutely summable in Q, and let g(x) be its sum. If there is a nonempty open subset U of P n Q such that f ( x ) = g(x) for any x E U, then f (x) = g(x) for any xEPnQ.

a.

Let u E U, and let u be any point of P n Q; then the segment joining u and u is contained in P n Q by (9.1 .I). Let h(t) =f(u + t(v - u)) - g(u t(u - u ) ) with t real; by (9.3.2), this is an analytic function of t in an open interval I

+

IX ANALYTIC FUNCTIONS

208

containing [0, 11. Let A be the closed subset of the interval [0, 11 consisting of the t such that h(s) = 0 for 0 < s < t ; by assumption there is an open neighborhood of 0 in [0, 11 which is contained in A, hence the 1.u.b. p of A is certainly > O ; we will prove that p = 1, which will establish (9.4.1). Note first that h(t) = 0 for 0 < t c p, hence by continuity h(p) = 0; as h is analytic at the point p, there is a power series in t - p, which converges for It - pi < a, with a > 0 and whose sum is equal to h(t) for It - pI < a. But for 0 < t < p, h(t) = 0 by assumption; from the principle of isolated zeros (9.1.5) it follows that h(t) = 0 for It - pI < a, which would contradict the definition of p if we had p < 1. (“Principle of analytic continuation”) Let A c KP be an open connected set, f and g two analytic functions in A with values in E. r f there is a nonempty open subset U of A such t h a t f ( x ) = g ( x ) in U, t h e n f ( x ) = g(x) for every x E A. (9.4.2)

Let B be the interior of the set of points x E A such that f ( x ) = g(x). It is clear that B is open and nonempty by assumption; we prove that B is also closed in A, hence equal to A since A is connected (see (3.19)). Let a E A be a cluster point of B; a s 5 g are analytic, there is an open polydisk P of center a, contained in A, such that in P, f ( x ) and g ( x ) are equal to the sums of two power series in the x i - a,, absolutely summable in P. But by definition, P n B contains an open polydisk U in which f ( x ) = g ( x ) . By (9.4.1) applied to P = Q, we conclude that f ( x ) = g(x) in P, in other words P c B, and in particular a E B. Q.E.D.

For p = 1, we can improve (9.4.2) as follows: (9.4.3) Let A c K be an open connected subset of K, f and g two analytic functions in A with values in E. Suppose there is a compact subset H of A such that the set M of points x E H for which f ( x ) = g ( x ) be infinite. Then f ( x ) = g(x)for every x E A.

Let (z,) be an infinite sequence of distinct points of M ; as H is compact, there is a cluster value b E H for the sequence (z”), hence any ball P of center b, contained in A, contains an infinity of points of M. But we can suppose f and g are equal to convergent power series in z - b in a ball P c A of center b ; the principle of isolated zeros (9.1.6) then shows that f ( x ) = g ( x ) in P, and we can then apply (9.4.2). For K = C we can also improve (9.4.2) in the following way:

4 THE PRINCIPLE OF ANALYTIC CONTINUATION

209

Let A c CP be an open connected set, f and g two analytic functions in A with vaIues in a complex Banach space E. Let U be an open subset of A, b a point of U, and suppose that f ( x ) = g ( x ) in the set U n ( b + RP); then f ( x ) = g(x)for eaery x E A.

(9.4.4)

We can suppose, by a translation, that b = 0; let h =f - g, and let P be a polydisk in CP, of center 0, contained in U and such that in P, h(z) is equal to the sum of an absolutely summable power series (cvzv).Now P n RP is a polydisk in RP,and h(x) = 0 in P n RP;this shows by (9.1.6) that c, = 0 for every vt hence h(z) = 0 in P and (9.4.2) can be applied. In an open connected set A c KP, we say that a subset M c A is a set of uniqueness if any two functions, defined and analytic in A, coincide in A as soon as they coincide in M. (9.4.2), (9.4.3), and (9.4.4) show that a nonempty open subset U of A, or the intersection U n ( b RP) (if not empty), or, for p = 1, a compact infinite subset of A, are sets of uniqueness. We shall see another example in Section 9.9 for K = C. The preceding result shows that if an open connected subset A c C p is such that A n Rp #fa, any analytic function f in A is completely determined by its values in A n RP.The restriction off to A n RPis an analytic function, but in general an analytic function in A n RP cannot be extended to an analytic function in A; we have however the weaker result:

+

Let E be a complex Banach space, A an open subset of RP,f an analytic mapping of A into E. Then there is an open set B c C p such that B n RP= A, and an analytic mapping g of B into E which extendsf.

(9.4.5)

Indeed, for each a = ( a , , . . . , up)E A, there is an open polydisk Pa in RP defined by Ixi - ail < ri (1 < i G p ) contained in A and such that,

in P,, f ( x ) is equal to the sum of an absolutely summable power series (c,,, . .,,,(xl - a,)”’. .. ( x p- a,)”p). Let Q, be the open polydisk in CP, of center a and radii ri; then, by (9.1.2), the power series (cn1... ,,,(z, - a,)”’ . . . ( z p- ap)”p) is absolutely summable in Q,; let g,(z) be its sum. If a, b are two points of A such that Q, n Qb # fa, then Pa n P b = (Q, n Qb) n RP is not empty, and we have g,(x) = & ( X ) =f ( x ) in Pa n Pb . Moreover Q, n Qb is connected by (9.1.1); it follows from (9.4.4) that g,(z) = &(Z) in Q, n Qb. We can now take B = Q,, and define g as ,

u

a

EA

equal tog, in each Q,; the analyticity of g follows from (9.3.1). The proof of (9.4.5) shows that when f is an entire function defined in RP, it can be extended to an entire function defined in Cp, and that function is unique by (9.4.4).

210

IX ANALYTIC FUNCTIONS

PROBLEMS

..

) a polynomial with coefficients in K, mi,. , m, elements of 1. (a) Let P(ul,. .. , u , + ~be K such that Iail < 1 for 1 < i < r . Suppose there exists a ball B in K of center 0 and a scalar function f analytic in B and such that f(z) = P(z,f(m~z), ,f(m,z)) for every z E B. Show that f can be extended to a function g analytic in the whole set K and satisfying the same functional equation in K (use (9.4.2)). and a scalar func(b) Suppose K = C, and suppose there is a real number tion f analytic for W ( z ) > P , and satisfying in that subset the equation f ( z ) = P(z, f(z +al), ,f(z a,)), where the a, are complex numbers with .%(ar)> 0. Show that f c a n be extended to a function g analytic in C and satisfying the same functional equation. (c) Generalize the preceding results to functions of any number of variables. 2. Let D be a connected open set in Cp,D' the image of D by the mapping

...

+

...

(21,

..., z,)

-+ (21,

..., 2 J .

Let f be a complex function analytic in D, and suppose D n Rp is not empty, D n D' is connected and f takes real values in D n Rp. Show that f can be extended to a function g analytic in D u D'. (Consider in D' the function (zl, .. ., z), -.f(il,.,fp),and use (9.4.4).)

..

5. EXAMPLES OF ANALYTIC F U N C T I O N S ; T H E E X P O N E N T I A L F U N C T I O N ; T H E NUMBER x

(9.5.1) Let P(z), Q(z) be two polynomials in KP, such that Q is not identically 0 ; then P(z)/Q(z) is analytic in the (open) set of the points z such that Q(z) # 0

(i.e., the set of points where the function is defined). It is obvious that any polynomial is an entire function. By (9.3.2) all we have to do is to show that l/z is analytic for z # 0; but if zo # 0, we can write 1

1

where the power series is absolutely summable for Iz - zoI < IzoI. Q.E.D. Consider now the function ex of the real variable x ; we prove it is an entire function. From the Taylor formula (8.14.3) we derive, for any n (using Section 8.8) e' d t .

5

EXAMPLES OF ANALYTIC FUNCTIONS

As ex is increasing by (8.5.3), we have Je'J < elx' for

JtJ ,<

211

1x1, hence

But if no is an integer > 1x1, we have

hence, for any x E R

and by (9.1.2) the series is normally convergent in any compact interval. Using the remark which follows (9.4.5), we can define in C an entire function ez (also written exp z) as equal to the sum of the power series (z"/n!). We have

(9.5.2)

ez+z'

- eZ& -

for both sides are entire functions in C2 which coincide in R2, and we apply

(9.4.4). For real x , e-'X is the complex conjugate of e'", since (-ix)" is the complex conjugate of (ix)";from (9.5.2)it follows that (eiXI= 1. We define cos x = B(e'"), sin x = 9(eiX) for real x; they are entire functions of the real variable x by (9.3.3), and the relation leixi = 1 is equivalent to cos2x + sin'x = 1, and implies lcos XI < 1 and Isin X I < 1 for any real x . Moreover, we have D(e') = ez

(9.5.3)

since both sides are entire functions (by (9.3.5) in C,which coincide in R. In particular (see Remark following (8.4.1)), D(eix)= ie'" for real x , hence

(9.5.4)

D(cos x)

=

-sin x ,

D(sin x ) = cos x.

The definitions of cos x and sin x for real x can also be written cos x = f(eix+ e -ix), sin x = (e'" - e-'")/2i; these formulas may be used to define cosz and sinz for complex z , replacing x by z in the right-hand sides. With these definitions, formulas (9.5.4)are still valid for complex values of x .

(9.5.5) There is a number II > 0 such that the solutions of the equation ez = 1 are the numbers 2nni (n positive or negative integer).

212

z

IX ANALYTIC FUNCTIONS

If z = x + iy, we have JezI = ex)eiyl= ex, hence ez = 1 implies x We first prove:

= 0,

= iy.

(9.5.5.1)

The set of points x 2 0 such that cos x

=0

is not empty.

One has

(4k

+ 3)(4k + 4)

and obviously the convergent series on the right hand side has all its terms 2 0; therefore

in other words, cos 2 < 0; therefore (3.19.8) the continuous function cos x takes the value 0 in the interval 10, 2[. As cos x is continuous, the set D of the roots of cos x = 0 such that x 2 0 is closed (3.1 5.1) and does not contain 0, hence has a smallest element which we denote by n/2. Then we have sin' 7112 = 1, and as sin x is increasing for 0 < x < 4 2 , sin 7112 = 1, einiz= i. This already shows that e21ri= 1, hence eZnlri= 1 for every integer n, and by (9.5.2) (9.5.6)

ezi2nni

- e'.

To end the proof of (9.5.5) we have only to show that the equation eix = 1 has no root in the interval 10, 2n[. But from(9.5.2) we deduce cos (x + 7~12)= -sin x, hence cos x d 0 for 4 2 d x < 71, and as cos (x + n ) = -cos x, we see that cos x < 1 for 0 < x < 2n, and this ends the proof. (9.5.7) The mapping x+ eix is a continuous bijection of any interval [a, a -t2711 on the "unit circle" U: IzI = 1 in C, and a liomeomorphism of la, a + 2n[ on the complement of eia in U. The mapping is obviously continuous, and it is injective by (9.5.2) and (9.5.5). To prove it is surjective in [a, a + 27-4, we can obviously suppose = ct + ip, a' + p' = 1 ; a = 0, for if [ E U , [e-'" is also in U. Let d 1 and, in the interval [0, 711, cos x is continuous and cos 0 = 1, as cos n = - 1, there is y E [0, rr] such that cosy = c( by Bolzano's theorem (3.19.8). Then sin y = & p ; if sin y = p, we are through; if not we have cos (2n - y ) = c o s y = c( and sin (271- y ) = -sin y = p. Let V be the complement of ei" in U, and TO = eibE V with a < b < a + 2n; if the inverse

EXAMPLES OF ANALYTIC FUNCTIONS 213

5

mapping of the restriction of x eix to ]a, a + 2 4 was not continuous at io, there would be in ]a, a + 2 4 a sequence (x,) whose elements would belong to the complement of a neighborhood of b, and such that lim eiXn= to; but then a subsequence (xllk)would tend to a limit c # b --f

m

in the compact set [a, a + 2x1 by (3.16.1), and as eic # eib we arrive at a contradiction. (For another proof, see (1 0.34.)

n-

(9.5.8)

The unit circle U is connected.

This follows from (9.5.7), (3.19.1) and (3.19.7).

(9.5.9) ("Principle of maximum") Let (c, z") be a porver series with complex coefticienfs,absolutelysummable in an open polydisk P c C p o jcenter 0 and let j ( z ) be its sum. Suppose that there is an open ball B c P ojcenter 0 such that If(z)l < If(0)l for every z E B. Then c, = 0 for every index v # (0, . . . , 0), in other tilords,f is a constant. We first prove that the theorem is true for any p if it is true for p = 1 . Indeed, for any z = (z,, . . . , zp)E P, consider the function of one complex variable g ( t ) =f( tz,, . . . , tz,) which is analytic for It1 < I E with E small enough. As Ig(t)l < Ig(0)J for small values of t, we have g(t) =g(O) by assumption, and in particular f ( z l . . . , z p ) = g ( l ) =f ( 0 ) . For p = I , we can suppose co 0, otherwise the result is obvious by (9.1.6). Suppose there are indices n > 0 such that cn # 0, and let m be the smallest of them. We can write

+

+

+

+

f ( z ) = ~ o ( l bmZm z " ~ ( z ) ) where b, # 0, h is analytic in P and h(0) = 0. Let r > 0 be such that IzI < r is contained in B and lh(z)l d +lb,l for IzI d r (9.1.3). Write b, = lb,lc with [[I = 1 ; by (9.5.7) there is a real t such that emit= [-';for z = reit, we therefore have 11

+ bmzm+ zmh(z)I= 11 + Ibmlrm+ zmh(z)I 3 1 + tlbmlrm

which contradicts the assumption If(z)l 6 IcoJin B. The result (9.5.9) does not hold if C p is replaced by RP, as the example

of the power series l/(l

+ z 2 )=

m

n=O

(- 1 ) " ~(for ~ " IzI < 1 ) shows.

214

IX ANALYTIC FUNCTIONS

(9.5.10) Let f be a complex valued analytic function dejined in an open subset A c CP, and which is not constant in any connected component of A. For any compact subset H c A, the points z E H where If ( z ) l = sup1f(x)I xeH

(which exist by (3.17.10)) are frontier points of H.

Follows at once from (9.5.9) and the principle of analytic continuation (9.4.1).

PROBLEMS

1. Show that if W(z) < 0, then, for any integer n > 0

2.

(use Taylor's formula (8.14.2) applied to t +P). Prove that, for real x

1

COS x -

(

1-

xz

+ x4 - . . . + (-

and the difference has the sign of (-

I)"+l;

)I 1X1Zn+2 +

1)" -

(X22 4n !

<(2n

2) !

similarly

and the difference has the sign of (-1)"x. (Use induction on n.) 3. (a) Let U be a relatively compact open subset of Cp, f a complex valued analytic function in U, which is not constant in any connected component of U. Suppose there is a number M > 0 such that for every frontier point x of U, and any E > 0, there is a neighborhood V of x such that If(z)l < M E for any z E U n V. Show that If(z)l < M for any z E U, and equality cannot be reached at any point of U (use (9.5.10) and the compactness of the frontier of U). (b) In C, let U be the open set defined by the conditions

+

W ( z )> 0, - 4 2 < 9 ( z ) < v/2.

Show that the entire function exp(exp(z)) is bounded on the frontier of U, but not in U. (a) Let E be the Banach space C2, with the norm ll(zl, z2)I/= sup(lzll, lzz\). The function z +f(z) = ( I , 0) (0, I)z is an analytic mapping of C into C2, such that Ilf(z)/l is constant for IzI < 1. (b) Extend the result of (9.5.10) to functions defined in an open set A c C p , and taking their values in a complex Hilbert space. (If Ilf(z)/l reaches a maximum at zo E A, consider the complex-valued function z + (f(z)lf(zo)); compare with (a).) 5. Let U be an open set in C p ,P a closed polydisk contained in U, of center a = (al, . .,a,) and radii r, (1 < k < p). Letfbe a complex valued analytic function in U, and suppose that on the set S = {(zl)\ (zl- all = r l for 1 =S i < p } (i.e., the product of the circles Iz, - all = rl), If(z)l < M. Show that for any z E P, If(z)l < M. (Use induction on p, considering the function (zz , . .. , z,,) +f(b,, z 2 , . . .,z,) for (bl - a , ( = rl.) 4.

+

.

5 EXAMPLES OF ANALYTIC FUNCTIONS

215

6. Let P(x, y) be a polynomial in two complex variables, with complex coefficients, of maximal degree m in x , n in y. Suppose that for real x , y such that - 1 < x < 1, -1 < y < 1, IP(x,y)l G M . Show that for real x , y such that 1x1 2 1, lyl > 1, IP(x,y)l < M(lx1 d-)"'(ly/ 4-Y. (Apply Problem 5 to the function

(i

s'"t"P

(s

+

:),+ :))

+

+

(t

for Is1 < 1, It I < 1 .) Extend to polynomials in any num-

ber of variables.

7. (a) Let f(z) be a complex analytic function of one complex variable in the disk B: IzI < 1 ; suppose If(z)l < M in B andf(0) = 0. Show that If(z)l < Mlzl in B (consider

the function f(z)/z, which is analytic in B) ("Schwarz's lemma"). When is equality possible ? for z = (z,, ,zp) (b) Consider on C p the norm llzll= (lzlIz .. . 1zp12)1/2, (called the " hermitian norm "). Let B be the ball /lzll< 1 for that norm, and let f be a complex valued analytic function in B, such that f(0) = 0 and If(z)l < M in B. Show that If(z)l < M llzll in B (consider the function t +f(zlt, . , z pt ) of one complex variable and use a)). 8. (a) In the complex field, let R- (the "negative real half-line") be the subset defined by 9 ( z ) = O,a(z) < 0; let F be the complement of R- in C. On the other hand, let S be the set defined by -2n < Y(z) < 2n. Show that the mapping z+ez is a homeomorphism of S onto F (use (9.5.7)); the inverse mapping is written z+ log z,and called the "principal determination of the logarithm of z"; one has log z = loglzl am(z) where am(z) is the unique number 6 such that - 2 n < 6 < 2n and z = lz/eiB (the "amplitude" of z). If z, z' and zz' are all in F, show that the difference log (zz') - log z - log z' is equal to 0,27ri or -22ni. is absolutely convergent; (b) In the ball B: jzI < 1, the power series ((-l)"z"/n). if f(z) is its sum, show that f ( z ) = log(1 z). (Observe that if z E B, 1 z E F ; show that f'(z) = 1/(1 z), and deduce from that result that f(z) = log(1 z) for z real and - 1 < z < 1;finally, consider the analytic function ef@)and use (9.9.4).) Conclude that log z is analytic in F.

+ +

...

..

+

+

+

+ +

(c) For any complex number t and any integer n > 0, let

(

where the cknare rational numbers we put (cknz"tk)is absolutely summable in

6x

(i)

=

1). Show that the power series

C (observe that for any number r > 0,

+

where a is a constant). Prove that the sum of that series is exp(t log(1 z)). (Consider first the case in which z and t are real, and apply Taylor's formula (8.14.2) to the function z + (1 z)'. Then use (9.4.4).) The function exp(t log(] z)) is also written (1 z)*; show that for real values o f t , 1(1 f z)'l= ( 1 zl'. (d) If t > 0, show that z + (1 z)' can be extended by continuity to the closed disk

+

Izl

< 1.

+

+

(Use a majoration of

for s > 0, 1 - s < e-s.)

+

+

similar to the one obtained in (c), observing that

216 9.

IX ANALYTIC FUNCTIONS

(a) Let f i k (1 < j < m, 1 < k < n) be scalar analytic functions defined in a n open connected subset A of Cp; let tcik be real numbers 30. Show that the continuous n

~fi~(~)~a'k~f~~(~)~'zk~f~~(~)~umk cannot reach a relative maximum at a point ofA,unless each of the products ~fIr(z)lu1"~~ Ifmk(z)lumk(l < k < n) is constant in

function u(z) = C

k=1

A. (Observe that if f(z) is analytic in A and f(zo) # 0, then, for every real number A, there is a function gA(z) which is analytic in a neighborhood of zo and such that IgA(z)j= If(z)lA in that neighborhood; use Problem 8(c) to that effect.) Extend the result to the case in which the tcJk are arbitrary real numbers, provided none of the hkvanishes in A. (b) Generalize to u(z) the result of Problem 3(a). 10. Let f ( z ) be a complex function of one complex variable, analytic in the open set A defined by R1< IzI < Rz (where 0 < R1 < Rz). For any r such that Rl < r < Rz,let M(r) = suplf(z)l. Show that if R1 < rl < rz < r3 < R , , then IT1 = r

log M(rz) <

log rz - log rl log M(r4 log r3 - log rl

r3 - log rz log M h ) + log log r3 - log rl

("Hadamard's three circles theorem".) (Apply Problem 9 t o I z I " . If(z)l, where the real number tc is conveniently chosen, and the function IzI' . f ( z ) is considered in the set r1 < IzI < r3 .) When can equality occur? 11. We put on Cp and Cq the hermitian norms (Problem 7). Let f be an analytic mapping of the ball B: llzll< 1 in C p ,into Cs;we havef= ( f l , . ,f,),where thef, are complex valued analytic functions in B. Suppose that f ( 0 ) = 0; show that if ilf(z)ll < M for z E B, then Ilf(z)l\ < M . llzll for z E B (for each z E B, consider the functions t +f,(tz)/t and apply Problems 9 and 3). When is there equality? 12. We put on Cp the hermitian norm (Problem 7). Let F, G be two analytic mappings of B: llz/l< 1 into C p ,which are homeomorphisms of B onto open sets U = F(B) and V = G(B), respectively, and such that the inverse mappings are analytic in F(B) and G(B), respectively (this last condition actually follows from the others; see Section 10.3, Problem 2). For any r such that 0 < r < 1, let B be the ball llzll < r, and let U, = F(B,), V, = G(B,), which are open subsets of U and V respectively. Show that if an analytic mapping u of U into V is such that u(F(0)) = G(O),then u(U,) c V,, for every r such that 0 < r < 1 (use Problem 11). 13. Let f be a complex valued analytic function of one complex variable in the ball B: IzI < R; for any r such that 0 < r < R, let A(r) = sup 9 ( f ( z ) ) .

..

lzl
(a) Show that r + A(r) is strictly increasing unless f i s constant (consider exp(f(z))). (b) Show that, when A(R-) < m,

+

A(r)

< -A(0) + -A(R-) R-r R+r

2r R+r

+

+

(Apply Problem 12, with F(z) = Rz, and G(z) of the type (az b)/(cz d), where the constants a, b, c, d are chosen such that G(B) is the half-plane defined by %(z) < A(R-).) 14. (a) Let A be a relatively compact open subset of C p ,E a closed subset of the frontier of A. Suppose there exists a complex valued functiong, which is analytic in a neighborhood of A, equal to 0 in E and is not identically 0 in any connected component of A. Let f be a complex valued analytic function in A, bounded in A, and suppose there is

6 INTEGRATION ALONG A ROAD

217

a number M such that, for every frontier point x $ E of A, and every E > 0, there is a neighborhood V of x in Cp such that If(z)l < M E for z E A n V. Show that If(z)l < M for every z E A. (One can suppose that jg(z)l < 1 for z E A. Consider the function If(z)l . I&)/', where a > 0 is arbitrary, and apply the result of Problem 9(b) to that function.) (b) Show that the result of (a) does not hold if the assumption that f i s bounded in A is deleted (consider the function exp(exp((1 - z)/z)) and use Problem 3(b)). 15. Let w(x) be a real function defined in [0, +a[, such that

+

w ( x ) > 0 and lim w(x) =

+ co.

x-fm

Show that if a complex-valued function f is analytic in a neighborhood of the closed half-plane A: W ( z ) > 0, then there is at least one point [ E A such that If((5)l < exp(w(l(/)lc/). (Use contradiction: if the conclusion was not true, prove would be < 1 in A, for every value of E > 0, by that the function lezl . If(~)l-~ applying Problem 9(a).) 16. Let A be an open relatively compact subset of C p , f a complex-valued function, analytic in A. Suppose there exists a number M > 0 and a complex-valued function g, analytic in A, such that g(z) # 0 for any z E A, and having the following property: for every point x of the frontier of A, and every E > 0, there is a neighborhood V of x such that If(z)/ < M Ig(z)Iefor z E A n V. Show that If(z)l < M in A (" PhragmtnLindelof's principle"; use Problem 9(b)). 17. Let U be the open set defined in Problem 3(b), and suppose f is a complex valued having the following properties: analytic function in a neighborhood A of (1) If(z)l < 1 on the frontier of U ; (2) there exists a constant a such that 0 < a < 1 and If(z)l < exp(exp(aW(z))) for z E U. Prove that lf(z)l < 1 in U. (Remark that

u,

1 Z+-

Z+l

transforms U into a relatively compact set, and use Phragmtn-Lindelof's principle (Problem 16) with g(z) of the form exp(exp(bz)).)

6. INTEGRATION ALONG A ROAD

A path in C is a continuous mapping y of a compact interval I = [a,b] 2 R, not reduced to a point, into C; if y(1) c A c C, we say that y is a pafh in A ; y(a) (resp. y(b)) is called the origin (resp. the extremity) of the path, both points are also called the extremities of y ; if y(a) = y(b), y is called a loop; if y is constant in I, we also say that the path y is reduced to a point. The mapping y o of 1 into C such that y(t) = y ( a +b - t ) is a path which is said to be opposite to y. Let I, = [b, c] be a compact interval in R whose origin is the extremity of I, and let I, = I u I, = [a, c]; if y , is a path defined in I,, and such that y,(b) = y(b), and if we define y z to be equal t o y in I, to y1 in I,, y, is a path which we denote y v y,, and which we call thejuxfaposifion of y and y,.

218

IX ANALYTIC FUNCTIONS

We will say that a path y, defined in I = [a, b] c R, is a road, if y is a primitive of a regulated function (8.7.2); if in addition y(a) = y(b) we will say that y is a circuit. It is clear that the opposite of a road is a road, and so is the juxtaposition of two roads. Let y , y 1 be two roads, defined in the intervals I, I, respectively. We say that y and y1 are equivalent if there is a bijection cp of I onto I,, such that cp and cp-' are primitives of regulated functions, and that y = y , 0 cp (hence y1 = y cp-'); it is immediate (by (8.4.1)) that this is indeed an equivalence relation between roads. If the road y is defined in I = [a, b], there is a road y, equivalent to y and defined in any other interval J = [c, d ] , for there is a linear bijection t --+ cp(t) = at + p of J onto I, and y, = y cp has the required properties. Let y be a road, defined in I = [a, b], and let f be a continuous mapping of the compact set y(1) into a complex Banach space E ; the function t -+f ( y ( t ) ) is then continuous in I, hence t -+f(y(t))y'(t)is a regulated function; the integral f(y(t))y'(t)dt is called the integral o f f along the road y 0

and written

s,

s%

f ( z ) d z ; from (8.7.4) it follows at once that if y, is a road

equivalent to y, then f ( z ) dz J-7, follows immediately that (9.6.1) (9.6.2)

s

=

s,

f ( z ) dz. Moreover, from the definition, it

s,.I(z) dz = f ( z ) dz

YlVYZ

s,

f(Z)

=S , , f ( z )

dz

dz

+ S,mdz

when the juxtaposition y1 v y z is defined. Let y be a circuit, defined in I = [ a , b ] ; for any C E I , consider the mapping y, of J = [c, c b - a ] defined as follows: y l ( t ) = y ( t ) if c < t < b, y l ( t ) = y ( t - b + a ) if b < t < c + b - a. It is immediately verified that y, is a circuit such that y,(J) = y(I), and that J Y , f ( z )dz = j Y f ( z )dz for any continuous mapping of y(1) into E. In other words, the integral o f f along a circuit does not depend on the origin of the circuit. Let y o , y1 be two paths defined in the same interval I, and let A be an open set in C such that yo(T) c A and yl(I) c A. A homotopy of yo into y1 in A is a continuous mapping cp of I x [a, p] ( a < j? in R) into A such that cp(t, c() = y o ( t ) and cp(t, p) = yl(t) in I; y 1 is said to be homotopic to y o in A if there is a homotopy of y o into y1 in A. It is clear that for any 5 E [a, b], t -+ cp(t, t) is a path in A. When both y o and y, are loops, we say that cp is a loop homotopy of y o into y1 in A if t -+ cp(t, t) is a loop for any 5 E [N, 83; when we say that two loops y o , y, are homotopic in A, we mean that there is a loop homotopy (and not merely a homotopy) of y o into y1 in A.

+

6 INTEGRATION ALONG A R O A D

219

If cp is a homotopy of yo into y1 in A, defined in I x [a, p], then the mapping ( t , 5 ) -+ q(t, a + p - 5 ) is a homotopy of y1 into y o in A ; on the other hand, if I) is a homotopy of y1 into y 2 in A, defined in I x [a’, p’], then we can define a homotopy 8 of y o into y 2 in A in the following ”p’ /?- a‘, we take way; we take 0 = cp in I x [a, p ] ; putting /I= 8(t, 5 ) = $(t, a’ - p) in I x [p, p ” ] ; this is meaningful, for both definitions give 8(t, p) = yl(t) by assumption, and it is immediate to verify that 8 is continuous in I x [a, p ” ] , takes its values in A, and is such that 8(t, a) = yo(?), O(t, p”) = y 2 ( t ) . This shows that the relation “ y l is homotopic to y o in A” between paths in A, is an equivalence relation; it is also an equivalence relation between loops in A, for the preceding definitions yield loop homotopies when cp and t,b are loop homotopies.

+

<+

(Cauchy’s theorem) Let A c C be an open set, f an analytic (9.6.3) mapping of A into a complex Banach space E . vrl, r2are two circuits in A which are homotopic in A , then f ( z ) dz = f ( z ) dz.

Sr

I

Sr

Suppose rl,r2are defined in I = [a, b ] , and let q be a homotopy of rl into r2in A, defined in I x [a, p] (N.B. It is not supposed that for 5 # a, p, the loop t -+ cp(t, 5 ) is a circuit). As cp is continuous, L = cp(1 x [a, 83) is a compact set contained in A ; by definition and the Borel-Lebesgue axiom, there exist a finite number of points a, (1 < k < m) in L and for each k an open ball P, c A of center a, such that: ( I ) the P k form a covering of L; (2) in each P,, f ( z ) is equal to the sum of a power series in z - a k , convergent in P, . There exists a number p > 0 such that for every x E L, the open ball of center x and radius p is contained in at least one of the P, (3.16.6). It follows from (9.3.1) that for every x E L, f ( z ) is equal in the ball B(x; p) to a convergent power series in z - x. As cp is uniformly continuous in I x [a, p] (3.16.5), there is E > 0 such that It - t’( d E , 15 - 5’1 < E imply Icp(t, <) - q ( t ’ , <’)I d p/4. Let be an increasing sequence in I such that to = a , fr = b, t i + l - ti < E , for 0 Q i < r - 1, ( ( j ) o s j s san increasing sequence in [a, 83 such that to= a, = p, < j + l - tj < E for 0 < j < s - 1. Define y j as follows

<

for t i Q t d t i + l , 0 d i < r - I , 1 <.j d s - I ; in addition, let y o = rl, ys = r 2 .Then y j is a circuit in A for 0 < j < s; all we have to do is to prove f ( z ) dz for 0 < j < s - 1. Note now that from the choice f ( z ) dz = L of the t i and t j , all the points y j ( t ) and yj+l(t), where t i d t d t i + l , belong

L,+

220

IX ANALYTIC FUNCTIONS

to the open ball Qij of center cp(ti, l j ) and radius p. By (9.3.7) and (9.3.1) there is a function g i j analytic in Q i j and such that gij(z) = f ( z ) in Q i j . As Q i - l , n Qij is not empty and is connected by (9.1 .I),the difference g i - l , - g i j is constant in Q i - l , j n Q i j by (8.6.1). Now, by definition r-1

f ( y j ( t ) ) y ; ( t ) dt =

C

i=O

i=O

r- 1

I

fi+l

gij(rj(t>>ri(t>dt

i

Therefore we are reduced to proving the relation r- 1

r- I

which can also be written r- 1

C (gij(yj(ti+

1))

i=O

- gij(yj+ l(ti+ 1 ) )

- gij(rj(ti>)+ g i j ( Y j + I ( t i > ) >

= 0.

But yj(ti) and yj+l(ti) both belong to Q i - l , n Q i j for 1 < i < r, hence, by what we have seen above gij(yj(ti)>- gij(Yj+l(ti>> = gi-1, j(yj(ti)) - gi-1, j(Yj+l(ti)) hence the left-hand side of (9.6.3.1) is reduced to gr-1, j(Yj(tr)) - gr-1, jbj+l(tr))- gOjbj(t0)) + 90j(yj+l(to)). But as y j and yj+l are circuits, we have yj(to)= yj(tr)and yj+l(to)= y j + , ( t r ) ; moreover, these two points belong to Q o j n Q r . - l , j , which is connected; the difference g r - l , - goj is thus constant in that set by (8.6.1), and this ends the proof. (9.6.4) Let yl, y 2 be two roads in an open set A c C,having same origin u and same extremity v , and such that there is a homotopy cp of y1 into y2 in A which leaves u and vfixed (i.e., q(a, I;) = u and cp(b, I;) = v for every I; E [a, /?I if cp is defined in [a, b] x [a,PI). Then, for every analytic function f in A,

j-p) dz j n f ( 4dz. =

+

Let yy be the road opposite to yl, and let y 3 ( t ) = yy(t - b a ) for b < t < 26 - a ; y 3 is a road equivalent to y y . By definition, y1 v y 3 and y 2 v y 3 are circuits. Moreover these circuits are homotopic in A, for if we define $ ( t , I;) as equal to cp(t, 5) for a < t < b, to y 3 ( t ) forb < t < 2b - a, $ is a loop homotopy in A. Applying (9.6.3), we get / y 2 f ( z )dz

+ jJ(4

dz. Q.E.D.

Yl

f ( z ) dz

+ JY, f ( z ) dz =

7 PRIMITIVE

OF

A N ANALYTIC FUNCTION

221

1

7. PRIMITIVE OF A N ANALYTIC F U N C T I O N IN A SIMPLY CONNECTED DOMAl N

A simply connected domain A c C is an open connected set such that any loop in A is homotopic in A to a loop reduced to a point; it is clear that any open subset of C homeomorphic to A is a simply connected domain.

Example (9.7.1) A star-shaped domain A c C with respect to a point a E A is an , segment joining a and z is contained open set such that for any ~ E Athe in A. Such a set is clearly connected ((3.19.1) and (3.19.3)); if y is any loop in A, write q ( t , 5 ) = a + (1 - t ) ( y ( t )- a ) for 0 < 5 < 1 ; 40 is a loop homotopy of y into the loop reduced to a. An open ball is a star-shaped domain with respect to any of its points. (9.7.2) If A c C is an open connected set, for any two points u, u of A there is a road of origin u and extremity u. We need only prove that the subset B c A of all extremities of roads in A having origin u is both closed and open in A (Section 3.19). If x E A n B, there is a ball S of center x contained in A, and by assumption S contains the extremity u of a road y of origin u ; the segment of extremities u, x is contained in S, and if y is defined in [a, b], the road y1 equal to y in [a, b], to y l ( t ) = u ( t - b)(x - v) in [b, b + 11is in A and has origin u, extremity x; hence x E B. On the other hand, if y E B, there is a ball S of center y contained in A ; for any u E S, the segment of extremities y , u is contained in S and we define in the same manner a road of origin u, extremity u, which is in A, hence S c B. Q.E.D.

+

(9.7.3) I f A c C is a simpIy connected domain, any function f analytic in A has a primitive Icjhich is analytic in A. Let a, z be two points of A, yl, yz two roads in A of origin a and extremity z; then J y , f ( x )dx = f ( x ) dx. Indeed, we may suppose, by replacing yz In by an equivalent road, that y1 is defined in [b, c] and y z in [c, d ] ; then y = yI v y y is a circuit in A, which is therefore homotopic to a point in A,

222

IX ANALYTIC FUNCTIONS

hence

j7f (x) dx = 0 by

Cauchy's theofem, and this proves our assertion.

We can therefore define g(z) as the value of f ( x ) dx for any road y in A J-7 of origin a and extremity z, and by (9.7.2), g is defined in A. Now for any z o E A, there is an open ball B c A of center zo in which f ( z ) is equal to a convergent power series in z - z,; by (9.3.7) there is therefore a primitive h off in B which is analytic, and such that h(z,) = g(zo); hence we have for ZEB

h ( z ) - h(z0) =

sd

f ( z o + t(z - z ~ ) ) (z zO) dt.

Jg

But the right-hand side is by definition f ( x ) dx, where CJ is the road t + zo t(z - z o ) defined in [0, 11; as that road is in B c A, we have g(z) - g(z,) = Jo f ( x ) dx by definition of g , and therefore g(z) = h(z) in B. Q.E.D.

+

8. INDEX OF A P O I N T WITH RESPECT TO A CIRCUIT

(9.8.1) Any path y defined in an interval I = [a, b] and such that y(1) is contained in the unit circle U = { z E C 1 ( z I = I}, has the form t -+ ei*(*), where $ is a continuous mapping of I into R; if y is a road, $ is a primitive of a regulated function.

As y is uniformly continuous in I, there is an increasing sequence of points t k (0 < k < p ) in I such that to = a, t, = b, and that the oscillation (Section 3.14) of y in each of the intervals 1, = [tk, tk+l] (0 < k < p - 1) be < 1. This implies that ?(I,) # U ; if 8, E R is such that eiek# y ( I k ) (9.5.7), then x + e'@' e k ) is a homeomorphism of the interval 10,274 on the complement of eiek in U (9.5.7). If q k is the inverse homeomorphism, we can therefore write, for t E I,, y ( t ) = eiJlk(*), where $k(t) = ( P k ( Y ( t ) ) +e, is continuous in I k . By (9.5.5), we have $k+l(tk+l)= $k(tk+l) + 2nkn with nk an integer (0 < k < p - 2 ) . Define now $ in 1 in the following way: $(t) = $ , ( t ) for t E 10; by induction on k, We put $(t) = $ k ( t ) $(tk) - $k(tk) for f k < t < tk+,. By induction on k , it is immediately seen that $ ( t k ) - $ k ( t k ) is an integral multiple of 271 for 0 < k < p - 1 ; therefore y ( t ) = ei*(') for t E I, and $ is obviously continuous in I. Moreover, if y(t) = a ( t ) + iP(t), we have a ( t ) = cos $ ( t ) , P ( t ) = sin $ ( t ) , and one of the numbers cos $ ( t ) , sin $ ( t ) is not 0; from (9.5.4), and (8.2.3) applied to one of the functions cos x, sin x at a point where it has a derivative # 0, we deduce that if y has a derivative at a point t , so has $, and i$'(t) = y'(t)/y(t),which ends our proof.

+

8 INDEX OF A POINT WITH RESPECT TO A CIRCUIT

223

(9.8.2) For any point U E C , and any circuit y contained in C - { a } , Jydz/(z- a ) has the form 2ntri, bvhere n is a positive or negative integer. By a translation, we can suppose a

= 0.

Suppose y is defined in I

=

[b, c ] ;

the function q(t, 5) = 5 y ( t ) + ( 1 - <)y(t)is continuous in I x [0, 11 and IY(t)l is a loop homotopy (in C* = C - (0)) of the circuit y into the circuit yl(t) = y(t)/ly(t)l, which is such that yl(I) c U. As l / z is analytic in C*, Cauchy’s theorem (9.6.3) shows that j y d z / z =/?, dz/z. But by (9.8.1), yl(t) = ei@(‘) where $ is a primitive of a regulated function, hence

jy,dz/z =

i jbc$’(t)dr = i($(c) - Il/(b)) by definition; as y,(b) = y,(c) by assumption, the conclusion results from (9.5.5). Remark. A simpler proof of (9.8.2), which does not use (9.8.1), can be

given as follows (Ahlfors): let h(t) =

s’ b

”(’) ds ; it has a derivative

dS)- a

equal to h’(t) = y’(t) except at the points of an at most denumerable YO) -a subset of I; hence, if g ( t ) = ePh(‘)(y(t) - a), we see that g‘(t) = 0 except in an at most denumerable subset of I. We conclude (8.6.1) that g is cont)-a stant, hence eh(‘)= y (. But we have y(c) = y(b), and therefore eh@)= 1, Y(b) - a which implies h(c) = 2n7ri for an integer n by (9.5.5). We say that the number n is the index of a with respect to y (or the index of y with respect to a ) and we write n = j ( a ; y). From Cauchy’s theorem it follows that if yl, y 2 are circuits in C - { a } which are homotopic in that set, they have the same index with respect to a. (9.8.3) The index j(x;y ) is constant in each connected component of the complement A of the compact set y(1). Indeed, we remark that x - t j ( x ; y) is continuous in the open set A, for by definition, the index of x + h with respect to y (if x h 4 y(1)) is equal to that of x with respect to the circuit yl: t + y(t) - h. But if B is a ball of center x and radius r, contained in A, q ( t , 5) = y(t) - l h (defined in I x [0, 11) is a loop homotopy, in C - {x}, of y into yl, as long as Ihl r, and therefore j(x + h ; y> = j ( x ; y ) by Cauchy’s theorem. As the set Z of integers is a discrete space, the conclusion follows from (3.19.7).

+

-=

224

IX ANALYTIC FUNCTIONS

Exainple

(9.8.4) Let c, be the circuit t -+ enitdefined in 1 = [0, 2n], M being a positive or negative integer; we have &,(I) = U ; E, is called “the unit circle taken n times”. We observe that the open set C - U has tii’o connected components, namely the ball B: IzI < 1 and the exterior E of B defined by IzI > 1. Indeed, B is connected as a star-shaped domain (9.7.1); and by Section 4.4and (9.5.7) E is the image of ]I, +a[x [0, 2n] by the continuous mapping (x, t ) -+ xe“, hence the result by (3.19.1), (3.20.16), and (3.19.7) (a similar argument also proves the connectedness of B and of B -{O}); finally in C - U, B and E are open and closed since B is open in C and B = (C- U) n and we have B n E = 0. From the definition and (9.5.3) it follows that j ( 0 ; E,) = n, hencej(z; E,) = n f o r any point z of B. Let us show thatj(z; cn) = 0 f o r any point of E; more generally:

s,

(9.8.5) Ifacivcuit yiscontainedinaclosedballD: Iz - a / < r, thenj(z; y ) = 0 f o r any point z exterior to D. Indeed, suppose y is defined in an interval I = [b, c], and that Iy’(t)l in that interval. By definition,


But as I y ( t ) - a1 < r, we have [ y ( t )- zI 3 [z - a1 - r for any t E I, and therefore, by the mean value theorem,

when Iz - a1 is large enough the right-hand side is < 2n, and as j(z; y) is an integer, this implies j(z; y) = 0. But the exterior of D is connected, as seen above, hence the conclusion by (9.8.3). (9.8.6) For any circuit y in C, defined in I, the set of points x E C - y(1) such that j(x; y ) # 0 is relatively compact in C. For by (9.8.5), that set is contained in any closed ball containing y(1). (9.8.7) Let A c C be a simply connected domain, y a circuit in A. For any point x of C - A , j ( x ; y ) = 0.

9 THE CAUCHY FORMULA

225

By assumption, there is in A a loop homotopy rp, defined in I x J, of y into a circuit reduced to a point. As x $ cp(1 x J), Cauchy’s theorem shows that dz/(z - x) = 0. JY

9. T H E C A U C H Y FORMULA

(9.9.1) Let A c C be a simply connected domain (Section 9.7), f an analytic mapping of A into a complex Banach space E. For any circuit y in A, defined in I and any x E A - ?(I), “e have (Cauchy’s formula)

Consider the function g ( z ) defined in A, equal to ( f ( z ) - f ( x ) ) / ( z - x) for z # x, to f ’ ( x ) at the point x; g is analytic in A, for it is obviously analytic in A -{x} by (9.3.2) and (9.5.1); on the other hand, there is a ball B c A of center x such that for z E B, f ( z ) = f ( x ) + ( z - x ) f ’ ( x ) + .. + ( z - x)’f‘”)(x)/n! ... the series being convergent in B; this proves that for any z E B, g ( z ) is equal to the sum of the convergent series

+

f’(x)

+ i ( z - x)f”(x) +

* * *

+ ( z - x)’-’f@)(x)/n! + . . .,

hence analytic at x. From Cauchy’s theorem (9.6.3) we have and writing

s,

g ( z ) dz

= 0,

yields (9.9.1) by definition of the index. Conversely : (9.9.2) Let y be a road in C, defined in an interval I = [b, c] and let g be a continuous mapping of y(1) into a complex Banach space E. Then f (z)=

s,gox x-z

is defined and analytic in the complement of y(1); more precisely, for any point a E C - y(I), if we write

the power series (c,(z - a)“) is convergent in any open ball B of center a contained in C - y(l), and its sum is equal t o f ( z ) in B.

226

IX ANALYTIC FUNCTIONS

Indeed, suppose Iz - a1 x E y(I), we have

1 -x-z

< q . d(a, y(1))

with 0 c q < 1 ; then, for any

=?

( ;I;) 1

(X-a)

with

I

if 6 = d(a, ~(1)).If IIg(x)11 tEI,

,,=O

1 --

( z - a)" ( x - a)"+'

< M in y(I)

1

( z - a) ( x - a)"+' '

1

5 q",

and Iy'(t)l

< m in I, we have, for any

hence the series of general term r'(t)s(r(t))(z

- a)"

( y ( t ) - a)"+

is normally convergent in I. It follows from (8.7.9) that the series (c,(z - a)") is convergent in the ball )z - a1 < q . 6 and has sumf(z) in that ball. (9.9.3) Under the assumptions of (9.9.1), we have, for every x E A - y(I), and every integer k > 0,

s

j ( x ; y)f'k'(x) = -

2ni k!

( zf (z )Xd)zk + l .

This follows at once from Cauchy's formula, the uniqueness of the coefficients of a power series with given sum (9.1.6), the relations (9.3.5) between these coefficients and derivatives, and finally (9.9.2). (9.9.4) Let A c C pbe an open set, f a continuous mapping of A into a complex Banach space E , such that for 1 < k < p , and an arbitrary point (a,) E CP, the mapping zk +f (al, . . . , a, - zk , a, + 1, . . . , up) is analytic in the open set A(a,, . . . , . . . ,up) c C i f that set is not empty (notation of (3.20.12)). Then f is analytic in A. More precisely, let a = (a,) be a point of A, P a closed polydisk of center a and radii r, ( 1 < k < p ) contained in A; for each k , let y k be the circuit t + a, + rkeit in C (0 < t < 2n), and let

',

9 THE CAUCHY FORMULA

227

0

Then the power series (c,(z - a)') is absolutely summable in P and its sum is equal to f ( z ) .

Using Cauchy's formula, and the fact that j ( 0 ; E have, by induction on p - k and the assumption, (9*9*4.1)

~= )

1 (see (9.8.4)), we

f(x1, . . . , x k , z k + l , * . * , z p )

for /xi - ail = rj (1 < j < k ) and Izj - ail < rj ( k + 1
- zp)

=c

(XI

- a,)"' * - - ( z , - a,)"" - U , ) " l + l ..-( x , - ap)n,+l

(2,

the power series on the right hand side being normally summable in the set F defined by Ixk - akl = rk (1 < k < p ) , by (5.5.3). By induction on p - k, if we write gn,+ ' ... n , ( X 1 >

* * *

7

Xk)

we have by the mean value theorem (9.9.4.2)

if

Ilf(xl,

. . ., x,)II


on

F. It

follows

that

the

power

series

(gn,+,. . . . p ( ~ l. ,. . , Xk)(Zk+l - a k + l ) n k + l*** (z, - a,)".) in the zj - aj is absolutely summable in P; using induction on p - k , and applying (5.3.5) and (8.7.9), we see that the sum of that series isf(x,, . ., xk, Z k + l , . . . , z,). The conclusion follows by taking k = 0. Moreover (9.9.4.2), for k = 0, proves

.

that, with the same assumptions and notations as in (9.9.4) (9.9.5)

Ilc,,,,, ...npll

< M/ri' . .

*

r;"

i f Ilf(x)ll < M on the product of the circles Ixk - akl = rk (1 (Cauchy's inequalities).


If in (9.9.4) we take A = Cp, we see that (9.9.6) An analytic mapping of C p into a complex Banach space is an entire function.

228

IX ANALYTIC FUNCTIONS

Observe that this last result is not true for analytic functions of real variables (1/(1 + x’) is a counterexample). Also, a continuous function f(x, y) of two real variables may be analytic in each of the variables without being analytic in R2;an example is given by f ( x , y ) = xy2/(x2 y’) for (x, v) f ( O , O ) , f ( O , 0 ) = 0.

+

Remark. It follows from (9.9.4) that the set F, product of the circles Ixk - a,l = r, (1 < k < p ) is a set of uniqueness in A (when A is connected); for the power series (c,(z - a)’) is entirely determined by the values off on F, hence if two analytic functions in A coincide in F, they coincide in P, and the result follows from (9.4.2).

PROBLEMS

1. Let A be a relatively compact open connected set in C. Let v be a continuous mapping of [a, b] x [0, 11 into A such that t-&, [) = y & ) is a circuit contained in A for 0 < [ < 1, and t -yo(t) = v(t,0) is a circuit contained in A (which may contain frontier points of A). Suppose in addition that for every E > 0, there exists 6 > 0 such that the relation /A - pi < 6 implies /y;(t) - y;(t)/ < E for t E [a,b] - D, where D is a denumerable subset. Let now f be a continuous mapping of A into a complex Banach space E, such

that its restriction to A is analytic. Show that Cauchy’s theorem j y o f ( z )dz

=IY,

f ( z ) dz

still holds (use (8.7.8)). 2. Let A be an open subset of C, f a continuous mapping of A into a complex Banach space E, such thatfis analytic in A n D, and A n D-, where D, (resp D-) is defined by 4 ( z ) > 0 (resp 4 ( z ) < 0). Show that f i s analytic in A . (Suppose the disk Izl < r is contained in A.Let y + (resp. y - ) be the circuit defined in [- I , + I ] by y + ( t ) = (2r 1)r for -1 < t < 0, y + ( t ) = r e f f i rfor 0 < t < 1 (resp. y - ( t ) = re”” for - 1 < t < 0, y - ( t ) = (1 - 2t)r for 0 < t < I.) Show that if Iz/ < r and 9 ( z ) > 0, then

+

using Problem 1 ; hence if y is the circuit t

--f

1 2nr

f ( z ) = -,

rezir in [- 1 , + I ] ,

jfE, x-z Y

Then use (9.9.2).)

3. Show that the conclusion of (9.9.4) still holds whenfis merely assumed to be bounded

in each bounded polydisk contained in A, but not necessarily continuous. (Use Problem 6 of Section 8.9; actually, a deep theorem of Hartogs shows that even this weakened assumption is not necessary; in other words, a function which is analytic separately with respect to each of t h e p complex variables ziis analytic in A,)

9

4.

m

Let f ( z )

THE CAUCHY FORMULA

a, z" be an analytic complex-valued function in the circle IzI

229

< R. Show

"=O

that, for 0 < r < R

Deduce from that result another proof of Cauchy's inequalities. 5.

Let f ( z ) =

m

a.z" be an analytic function in IzI

< R, and let

"=O

m

M ~ ( r ; f )= Let also M(r;f)

=

C llanllr".

"=O

sup Ilf(z)ll. 111 = I

(a) Show that for 0 < r


+6 < R

(use Cauchy's inequalities). (b) If in addition, f i s complex valued, show that (with the notations of Problem 4)

(Use the Cauchy-Schwarz inequality (6.2.1).) (c) Under the same assumption, show that lim (Ml(r; f"))""

"-m

=

lim (Mz(r; f"))""

"-m

=

M(r; f)

-

(use the inequalities proved in (a) and (b), the fact that M(r; f") = (M(r; f))",and the continuity of r M(r; f)). 6. Suppose the power series in one complex variable (c,z"), with complex coefficients, is convergent for Iz/ < R , and let f ( z ) =

c.z". n

A(r) = sup %(f(z)). Show that, for every n IzI=r

lcnlr"

For any r such that 0 < r < R, let

>0

+ 2 9 ( f ( O ) ) < sup(4A(r), 0).

(Prove that

for n

> 0.)

7. (a) Let A be an open subset of Kp, and f a n indefinitely differentiable mapping of A into a Banach space E. In order thatfbe analytic in A, it is necessary and sufficient that for every compact subset L of A, there exist an integer r 3 0 and a number a > 0 such that, for any index a = ( a 1 ,. . . , a,,), sup lID=f(x)l/ < a(la1 r ) ! . (To prove that the X E L

+

condition is necessary when K = C, apply Cauchy's inequalities to balls of fixed radius contained in A and having their centers in L ; when K = R, use (9.4.5) ; to prove that the condition is sufficient, use Taylor's formula (8.14.3) and prove that the last term of that formula tends to 0 uniformly in any closed ball contained in A and of center x . )

230

IX ANALYTIC FUNCTIONS

(b) Give an example of an indefinitely differentiable function in R which is not analytic (cf. Section 8.12, Problem 2). (c) Supposefis real valued and indefinitely differentiable in an open interval I C R; in addition, suppose that there is an integer p > 0 such that f("'does not vanish at more than p points of I, for any n > 0. Show that f is analytic in 1. (Use (a), and Problem 3(b) of Section 8.12).

10. CHARACTERIZATION O F ANALYTIC F U N C T I O N S O F COMPLEX VARIABLES

A continuously differentiable mapping f of an open subset A of C p into a complex Banach space is analytic.

(9.10.1)

Applying (9.9.4), we are immediately reduced to the case p = 1. To prove f is analytic at a point U E A ,we may, by translation and homothetic mapping, suppose that a = 0 and that A contains the unit ball B: IzI < 1. For any Z E B, and any I such that 0 Q I Q 1, note that 1(1 - I ) z + Iei'l < 1 - I + I = 1, and consider the integral (9.10.1 . I )

By (8.1 1.I) and Leibniz's rule (8.1 1.2), g is continuous in [0, 11 and has at each point of 10, 1[ a derivative equal to g ' ( I ) = Joznjr(z + I(eit - z))ei' dt

(see Remarks after (8.4.1)). But i y ' ( z + I(eif - z))e" is the derivative of t +f(z + i(ei' - z)), hence, for I # 0, g ' ( I ) = 0, and therefore (remark following (8.6.1 )), g is constant in [0, 11. But asg(0) = 0, g ( I ) = 0 for 0 Q A Q 1. In particular, it follows, for I = 1, that

for any z E B (by (9.8.4)), and the conclusion follows from (9.9.2).

Let f be a continuously dlfferentiable mapping of an open set A c RZPinto a complex Banach space. In order that the function g dejined in A (considered as a subset of C p ) , by f ( x l , x 2 , .. ., x p , y l , . ., y,) = g (xl + iyl, . . . , x, + iy,) be analytic in A, necessary and suflcient conditions are that (9.10.2)

in A for 1 < k

< p (Cauchy's conditions).

10 CHARACTERIZATION OF ANALYTIC FUNCTIONS

231

We are again at once reduced to the case p = 1 by (8.9.1). Let (x,y )

df (x,y ) , b = af (x,y ) ; expressing that the be a point of A, and put a = ax 8Y limits lim (g(x iy 12) - g(x + iy))/h and lim (g(x + iy + ih) - g(x iy))/ih

+ +

+

h-+O

( h real and ZO) are the same, we obtain a + ib = 0. Conversely, if that condition is satisfied, for any E > 0, there is r > 0 such that if (h2 + k 2 ) 1 / 2< r, 11g(x iy h + ik) - g(x iy) - a(h + ik)ll < e(h2 + k 2 ) 1 / 2by (8.9.1 .I)and this proves that z -g(z) has a derivative equal to a at the point z = x iy. The result then follows from (9.10.1). h-tO

+ +

+

+

PROBLEMS

1. Show that a differentiable mapping o f f an open subset A of C p into a complex Banach space is analytic in A ("Goursat's theorem"; f' is not supposed to be continuous). (Given any h in ]0,1[, prove (with the notations of (9.10.1)) that g'(h) exists and is equal to 0. First show that, given E > 0, there are points t o = 0 < t l < . .. < 1, = 2n, a number p > 0, and in each interval [ t k , t k + 11 a point 8, such that, if ( k = z &elek - z), whenever (k x =z ( A h)(e" - z), then x) -f((k) -f'((k)xl < & / X I Ihl < p and tk < t < t k + l (prove this by contradiction, using a compactness argument and the existence off' at each point). Compare then each integral

+

+ +

+

+

to the expression

2.

for Ihl < p.) Let A be an open simply connected subset of C; i f f is a continuous mapping of into a complex Banach space E such that

s,

f ( z ) dz = 0 for any circuity in A, show that f i s

analytic in A. (" Morera's theorem"; show that f h a s a primitive in A.) 3. Let A be an open subset of Cp, y a road defined in I = [a, b], f a continuous mapping of y(1) x A into a complex Banach space E. Suppose that for each x E y(I), the function ( z l , . .. ,z,) + f ( x , zl, . .. , z,) is analytic in A, and that each of the functions

af -(x,

.

z l , . , ,z,) is continuous in y(1) x A (1 < k

conditions, the functions g(zl,. .. , z,)

=


Show that under these

( f(x, ...,z,) dx is analytic in ZI,

JY

A. (use

(9.20.2). Asy'(t) is merely a regulated function and may fail to be continuous, Leibniz's rule (8.11.2) is not directly applicable, but the proof of (8.11.2) subsists with minor modifications.) 4. Let A be an open connected subset of RP(p > 2 ) , f a n analytic mapping of A into a complex Banach space E. Suppose that there is an open polydisk P C A, of center b= < k < p and radii rL (1 < k < p ) such that for every point (ck) of P, there is a

232

IX ANALYTIC FUNCTIONS

+

number p < inf(rl, r 2 )such that the function x1 ixz +f(xl, xz , c 3 ,.. . , cp)is analytic in the open subset /xl ixz - (cI icz)l < p of C (identified to R2).Show that the same property holds for every point (ck) E A (use (9.10.2) and (9.4.2)). 5. Let S be the "shell" in RD( p 2 3) defined by

+

+

+ XI+ '. + x', < (R +

(R - E)' < X :

E)'

(0 < E

< R).

Suppose f is an analytic mapping of S into a complex Banach space E, and suppose that for any u = (x3, .. ,x p ) , the mapping x1 ixz + f ( x l , x 2 , u) is analytic in a neighborhood (in C) of every point of the cross section S(u) (if S(u) is not empty). (a) For any u = ( x 3 ,. .., x,) such that lluIl2 = x: . . . x', < RZ,let y(u) be the road in C defined by f +(RZ - IIuljZ)l/zeiffor -7r < f < T.Let

+ + +

.

+

+

where y = x1 ixz , and f ( y , u ) =f(xl, xz, u ) ; g is defined for lzlz l/ull2< RZ,and z+g(z,u)isanalyticforlzl < (R2 - jl~ll~))'/~.Ontheotherhand,foranyv = (x;,. . . , x i ) such that Ilu II < R. let

+

Show that h,(z, u ) = g ( z , u ) for llull < l l ~ l< l llull E and /z /cc (R2- l l ~ l l ~ ) (apply '/~ Cauchy's theorem (9.6.3)). On the other hand, show that g(z, u ) = f ( z , u) for R - E < llull < R and /zI < (RZ- I I U I ~ ~ ) ' ~ ~Conclude . that f can be extended to a function f which is analytic in the whole ball B : x i . . . x: < (R E ) ~(apply (9.4.2) and Problem 3). Is the theorem still true for p = 2 ? (b) When E = C, show that f(B) c f ( S ) . (Apply the result of (a) to the function l/(f- c), where c $f(S).) In particular, iffis bounded in S , f i s bounded in B. Extend that last property to the case in which E is a complex Hilbert space (method of Problem 6 of Section 8.5).

+

+ +

11. LIOUVILLE'S THEOREM

(Liouville's theorem) Let f be an entire function in C p , with values in a complex Banach space E. Suppose there exists two numbers a > 0, N > 0 such that ]]j(z)I] < a (l ( ~ u p l z ~ lin) ~C". ) Then f ( z ) is the sum ofajinite with ~ c,, . . . n p E E and number of "monomials" C , , , , , ~ . ....~. :2~ Z ~ (9.1 1 . I )

+

nl

Let f ( z ) =

+ n2 + + np < N. **.

cvzv in C p , the power series being everywhere absolutely

convergent. The Cauchy inequalities (9.9.5) applied to the polydisk lzj]< R (1 < j < p ) yields, for any v = ( n l , . . . , np) Q a * (I R ) ~ R - ( " , +... + ' p ) . lc,,,

+

Letting R tend to

+ co, we see that c,, ...",

= 0,

unless n,

+ + np < N .

12 CONVERGENT SEQUENCES OF ANALYTIC FUNCTIONS

233

(9.11.2) (The "fundamental theorem of algebra"). Any polynomial f ( z ) = a,z" + a,zfl-l + * . . + a,, (a, # 0, n 2 1 ) with complex coeficients has at least one root in C. Otherwise, l / f would be analytic in C (9.3.2), hence an entire function (9.9.6). Let r be a real number such that rk 2 ( n t l ) ~ a k / afor o ~ 1 d k dn; then, for IzI 2 r

In other words, l / f is bounded for lzl 3 r. On the other hand, l / f being continuous in the compact set IzI < r, is also bounded in that set (3.17.10), hence l / f is bounded in C.Liouville's theorem then implies l / f is a constant, hence also f, contrary to assumption since If(z)I 2 la,] . Izl"/(n + 1 ) for IzI 3 r.

PROBLEMS

If p > 2, show that a function which is analytic in the complement of a compact subset of Cp is an entire function; hence if in addition it is bounded in the complement of a compact subset of C', it is a constant (use (9.1 1.I) and Problems 4 and 5 of Section 9.10). Is the result true for p = 1 ? L e t f b e a complex valued entire function in CP. Show that the conclusion of (9.11.1) is still valid if it is supposed that

Wf(z)) < a * (sy41,Iz,l)'") for any z in the exterior of a polydisk of CP (use Problem 6 of Section 9.9). Let f(z)

m

= "=O

a, z" be a nonconstant

M(r) = sup Ilf(z)II, so that p(r) IZI =I

entire function. For any r

< M(r);

> 0, let p(r) = sup JJanJIr", n

by Liouville's theorem, lim p(r)= f a . I-m

Suppose there are two constants a > 0 , a > O such that p ( r ) < a.exp(r"); show that there are positive constants b, c such that M(r) < brap(r) c. (Observe that llanIl< a(ea/n)"".)

+

12. CONVERGENT SEQUENCES O F ANALYTIC F U N C T I O N S

(9.12.1) Let (f,)be a sequence of analytic mappings of an open set A c C p into a complex Banach space E. Suppose that for each z E A, the sequence (f,(z)) tends to a limit g(z), and that the convergence is uniform in every compact

234

IX ANALYTIC FUNCTIONS

subset of A, Then g is analytic in A, and f o r each v = ( nl, . . . , np)E NP, the sequence ( D v L ( z ) )converges to Dvg(z)f o r each z E A, the convergence being uniform in every compact subset of A. As g is continuous in A (7.2.1), to prove g is analytic in A, we need only prove that each mapping zk +g(a,, . . . , zk, . . . , up) is analytic in A(a,, .. . , a k - 1 , a k f l , . . . , up), by (9.9.4); in other words we are reduced to the case p = 1. For each a E A c C, let B be a closed ball of center a and radius r contained in A, and let y be the circuit t + a reir (0 < t d2n); then, for each z E fi and each n, we have by Cauchy’s formula

+

But by assumption the sequence ( f , ( x ) ) converges uniformly to g(x) for Ix - a( = r, and as ) z - x( 2 r - IzI, the sequence (L(x)/(x- z ) ) ( z fixed) also converges uniformly to g(x)/(x- z ) for Ix - a ) = r ; hence, by (8.7.8)

which proves g is analytic in

fi by (9.9.2).

Moreover, as

by (9.9.3), the same argument (and (9.9.3) applied to g) shows that f ’ ( z ) tends to g’(z) for every z E 8; furthermore, we have by the mean-value theorem

Returning to the general case ( p arbitrary), let us now show that the sequence ( D k f , ( z ) )converges uniformly to D k g ( z ) in any compact set M c A. There is a number r > 0 and a compact neighborhood V of M contained in A, and containing all points of A having a distance d r to M (3.18.2). For any E > 0, let no be such that \lg(z) -f,(z)l) d E for every n > no and every z E V. Then, applying (9.12.1 . I ) to the sequence of functions zk -+&(al9. . . , ak- zk, ak+ . . . , up), we obtain, for every point z E M, (IDkg(z)- Dkffl(z))I< E/r as soon as n 2 n o . This ends the proof of the theorem when n, + * + np = 1 ; the general case is then proved by induction on nl np.

,,

+ . - -+

12 CONVERGENT SEQUENCES OF ANALYTIC FUNCTIONS 235

Observe again here that the theorem does not hold for analytic functions of real variables, since a sequence of polynomials can have as a limit an arbitrary (e.g. nondifferentiable) continuous function in a compact set, by the Weierstrass approximation theorem (7.4.1).

PROBLEMS

1. (a)

Let

(ak)l,
be a finite sequence of complex numbers, such that

P

C lakl = a < 1 . k=l

Show that

(b) The entire functions E(z, 0) = 1 - z,

E(z,p) = (1

- z ) exp

are called primary factors; show that, for Iz/ < 1/2 IE(z,p)

-

11 < 4/zlP+'.

(Observe that for Iz( < 4 log(1 - z)

+ z f -2 + . + P ZZ

' '

zp

I

< 2IzlP+'/(p + 1 )

and that for IzI < 1 , lez- 11 < 2121.) (c) Let (an)be an infinite sequence of complex numbers # O , such that the sequence (1a.l) is increasing and lim lan\= co. Show that for any z E C , the series of general n-r m

+

term (z/a.)" is absolutely convergent. (d) Deduce from (a), (b), and (c) that the sequence of entire functions p.(~)=nE -,k-1 k= 1

( i k

)

is uniformly convergent in every compact subset of C (apply Cauchy's criterion, and evaluate the difference 1 - (p,(z)/p.(z)) for m > n by using (a) and (b); then apply (c)). The limit f(z) of the sequence (p.(z)) is thus an entire function, which is written

show that the only points wheref(z) = 0 are the points a, (use the preceding estimate). ~-p (e) Suppose that there is an integer p > 0 such that the series of general term ~ u m is convergent. Show similarly that the sequence of entire functions

236

IX ANALYTIC FUNCTIONS

is uniformly convergent in every compact subset of C; its limit is again written

Prove that there is a constant c > 0 such that lg(z)l < exp(c izl'). (For any given z, consider separately the product of the factors for which la,, 2- 2 Iz/, and of the other factors; use (b) to niajorize the first product; on the other hand, prove that there is a constant b such that IE(z, p - 1)1 < exp(6 /zip-') for any z E C.) 2. Show that the sequence of entire functions fn(z) = z(z 1) . ' ' ( z n)/n* N ! (1)

+

+

(where tiz = exp(z log n) by definition) is uniformly convergent in every compact subset of C to the entire function

(" Euler's constant "). (Use the result of

log(k/(k - I)) to compare (I) and (2), and using the

Problem I(e), writing log IZ =

mean value theorem to majorize - - log Prove that

r(z)satisfies the functional equation r(z+ 1) = zr(z)

when z is not an integer -n < 0, and that l'(rr) = ( n - I ) ! for n integer and > O . 3. An endless road in an open subset A C C is a continuous mapping y of R into A such that in every compact interval I C R, y is the primitive of a regulated function. Iff is a continuous mapping ofy(R) into a complex Banach space E,fis said to be improperly integrable alongy if the improper integral

j. f ( y ( t ) ) y ' ( t )

c//

-an

exists (i.e., if both limits

lim /aof(y(/))y'(r) dt exist in E); the value of that -m

lim Jobf(y(t))y'(t) dt and b-fm

a+

integral is then callled the integral off dong y and written J y f ( z )dz. Let B be an open subset of C p ,g a continuous mapping ofy(R) x B into E; suppose that for each x E y(R), the function ( Z I , . . , z,) -+g(x,zI,. . . , 2,) is analytic in B and % that each of the functions - (x,z , , . . . , z,) is continuous in y(R) x B. Finally

.

suppose that for each (zl, . .. , z,) alongy,and that

j

'"

g(y(t), z,,

E

.

B, x + g ( x , zl, . . , z),

. . .,z,)y'(t)

c//

is improperly integrable

tends uniformly to

-n

s,

y(x, z I , . . . , z),

ctx

when (zI,. . . , z,) remains in a compact subset of B and n tends to i-co. Under these conditions, show that the function ( z I ,. . . , z,) 4.

+

ij

g(x,zl,

. . . , z,)

dx is analytic in B

(compare to (13.8.6)). Extend the result of Problem 2 of Section 9.9 to functions of p complex variables, D, (resp. D-) being defined by S(z,) > 0 (resp. .P(z,) i 0). (Observe that, by (9.12.1), for each z, such that .P(z,) = 0 and the intersection B of A with the set C p - ' x (z,) is not empty, the function (zI, , , z P - J+f(zI, . . . , z , - I , 2,) is analytic in B.)

..

13 EQUICONTINUOUS SETS OF ANALYTIC FUNCTIONS

5.

237

In the plane C, let Q be the square of center 0, defined by lg(z)l < I , l.Y(z)l < I . Let Q o , Q1, Q 2 , Q 3 be the images of Q by the mappings I + i

z

2

2'

z+---+-

ZJ-

-l+i 2

+-2'z

z+-

-1-i 2

I-i 2

z

f-, z+-+-. 2

+ + +

+

z

2

Let m,, = 0 , and for any h > I , let mh = 4 4' . . . 4h; if n = mh 4k + j , with h > I , 0 < k < 4h - I , 0 < j < 3, define inductively Q, as follows: let nl = mh-1 k, and let zn1be the center of Q,,; let p.,(z) z., -t- z/2* and take Q. = y,,,(Q). (a) Let B be the unit disk /zI C I , U the unit circle IzJ= 1. Show by induction on n the existence of three sequence of numbers (a"),(cJ, (t,) defined for I I 2 4, having the following properties:

+

~

(I)

O
(2) if g.(z)

and fn(z)

=

( i T"1 6 +x

= c,

z

1- 1--

(definition in Section 9.5, Problem 8) for z E B,

g,(z), then fn(B) c

and J,(tk) E Qr for k

< n;

9=4

(3)

the seriesx 1c.I is convergent. n

(Observe that gn(fn)= c,, but that, given any neighborhood V, oft. in B, it is possible to take a, small enough so that gn(z)will be arbitrarily small in B - V.. Choose t , close to t., (with the notations introduced above), the t, being all distinct, and take V. so that it contains no t k with k < n.) (b) Under the preceding conditions, the limit f ( z ) of ( h ( z ) ) exists for any z E B, f i s continuous in B, and analytic in 6, and f(U) = Q (" Peano curve," cf. Section 4.2, Problem 5).

13. E Q U I C O N T I N U O U S SETS O F ANALYTIC F U N C T I O N S

Let A be an open set in C p ,4, a set of analytic mappings of A into a complex Banach space E. Suppose for each compact subset L of A, there is a constant mL > 0 such that 11 f(z)ll d mL for all f E 4, and every z E L. Then @ is equicontinuous in A (Section 7.5); f i n addition E isJinite dimensional, then for every compact subset L of A, the set QL of restrictions to L of the functions f E @, is relatively compact in the space %,(L) (Section 7.2). (9.13.1)

Let a E A; there is a closed ball P c A of center a, radius r, and as P is compact, Ilf(z)ll
IX ANALYTIC FUNCTIONS

238

Now f ( z l , ..., z , - ~ , zk, a k f l r..., u p ) - f ( z , ,

.

= j o 1 ~ , / ( z 1 ,. . , z k - 1 , ak

..., zk-1,

+ t(z, - ak),

a,,

...,a

. . ., ag)(zk- a,>

p)

dt.

Write gk(U) = f ( z l , . . . , Z k - l , u, . . . , a,,); gk is analytic in an open set of C containing the ball Iu - akl < r, and IIgk(U)(/< mp in that ball. Applying (9.9.3) to gk and to the circuit t .+ ak + reir defined in [O, 2x1, we obtain

Ils;(u>ll

< 4mdr

for Iu - akl < r/2. Therefore, for any z

E

Q , and any f

E

CD we have

which shows CD is equicontinuous at the point a. The last statement of (9.13.1) follows from the fact that any bounded set in a finite dimensional space is relatively compact ((3.17.6) and (3.20.17)), and from Ascoli’s theorem (7.5.7).

(9.13.2) Let A be an open connected set in C p , CD a set of analytic mappings of A into a complex Banach space E. Suppose for each compact subset L of A, the set CDL of restrictions to L of the functions f E CD is relatively compact in eE(L). If M is a set of uniqueness (Section 9.4) in A, and if a sequence (f.)of functions of CD converges simply in M, then (f.)converges uniformly (to an analytic function) in any compact subset of A.

From (3.16.4) it follows that we need only prove that, for every compact set L c A, the sequence of the restrictions of thef. to L has only one cluster value in WE(L). Suppose the contrary, and let (g,,), (11,) be two subsequences of (f.),each of which converges uniformly in L, the limits being distinct. As A is locally compact (3.18.4) and separable, there exists an increasing sequence (U,) of open subsets of A, such that 6, (closure in C”) be compact and contained in U n + l ,and A = U, (3.18.3). Define by induction on k

u n

a sequence (gkn)n=l,2,... , such that (gk,,) is a subsequence of (gk-l,,), with go,,= g, , and that (gkn)converges uniformly in U, , which is possible by the assumption on CD. Then the “ diagonal subsequence (gn,) converges uniformly in every U,, hence, by (9.12.1) its limit g is analytic in A. In a similar way it is possible to extract from (h,) a subsequence (A,,,,) which ”

14 THE L A U R E N T SERIES

239

converges in A to an analytic function h. Now by assumption g ( z ) = h ( z ) for z E M , and by definition, we must have g = h. But this contradicts the definition of the subsequences (g,,), (A,,). Q.E.D.

PROBLEM

Let A be an open set in C, and let E be the set of all complex functionsfanalytic in A and such that the integral

If(z)l* d x dy is finite.

Show that E is a complex vector space, that for any pair f,g of functions in E, the integral

lJkJ(z)z)

d x dy is finite and that the mapping (f,g ) +

/Lf(

z)g(z)d x dy

defines on E a structure of prehilbert space, whose norm is written l l f l l . Show that for any compact subset L c A, there exists a number uL such that If(t)l C aLllfll for every function f c E and every t E L (use Section 9.3, Problem 6). Conclude that E is a Hilbert space. Deduce from (b) and from Section 6.3,Probleni 5, that there exists in E a reproducing kernel, which is called the Bergman kernel of A. Prove that if A is the unit disk Iz/ < 1, the Bergman kernel of A is 1 Ke(s, t ) = 7r( 1 - si)2 ~

(use Section 9.3, Problem 6). Compute similarly the Bergman kernel of a ringr < IzI < 1 . Generalize to an open subset A of C”.

14. T H E L A U R E N T SERIES

(9.14.1) Let A he an open subset of C, y o , rl two numbers such that 0 < ro < r l , and suppose the “open ring” S dejined b y ro < IrI < r1 is such that its closure in C (i.e. the “ closed ring ro < IzI < r,) is contained in A. For any analytic mapping f of A into a complex Banach space E, we have, f o r any x E S

s

”

\ihere y o (resp. yl) is the circuit t -+ roeit (resp. t + r l e i f )with O Q t Q 27c. As in the proof of (9.9.1), we first see that the function g ( z ) equal to f ’ ( x ) at the point x and to ( f ( z ) - f ( x ) ) / ( z - x) for z # x, z E A, is analytic in A. Now, q(t,5) = [ r o e + i r ( l - ()rleir (0 Q t Q 27c, 0 < ( Q 1) is a loop homotopyin Aof yo into y1 ;hence /yog(z)dz = / y l g ( z )dz by Cauchy’s theorem (9.6.3). But for ro < 1x1 < r l , we have ,j(x; yo) = 0 and j(x; yl) = 1 ((9.8.4) and (9.8.5)), hence the result.

240

IX ANALYTIC FUNCTIONS

(9.14.2)

Under the same assuniptions as in (9.14.1), there exists a power m

1 cnzt1,convergent f o r I

series g,(z) =

n=O

iisithout constcrnt term, g 2 ( z ) =

IzI < r l , and a poii'er series in l/z and

m

C dnz-", convergent f o r

IzJ > y o , suclz that

n= I

f ( z ) = g,(z) + g 2 ( z ) in S ('' Laurent series" off). Moreover the poii'er series g l , g , hacing these properties are uiiiqzie, and, .for every circuit y in we h a w

s,

By (9.9.4) we have

the series being convergent for Iz( < r l . On the other hand, for IzI > y o , 1x1 = y o , we have y- 1 1 -z - x - 2 z" where the right-hand side is normally convergent for 1x1 = ro ( z fixed); by (8.7.9), we get

the series being convergent for IzI > r o . This proves the first part of (9.14.2). Suppose next we have in S f ( z ) = C a n z n+ U,

(9.14.2.1)

n=O

C bflz-" n;

n= 1

both series being convergent in S ; let first y be a circuit in S, defined in I; there are points t , t' in I such that y ( t ) = inf y(s) = r and y ( t ' ) = sup y ( s ) = r' ssl

S€l

(3.17.10), hence ro < r < y(s) < r ' < r1 for any s E I . But, for r < JzI< r ' , both series in (9.14.2.1) are normally convergent (9.1.2), hence by (8.7.9), for any positive or negative integer in

+

As zk"/(k I ) is a primitive of z' for k # - 1, we have / / k dz = 0 for any circuit a ; (9.14.2) then follows from the definition of the index. If now y is in 5, we remark that there is an open ring S , : ( 1 - E)ro < IzI < ( I + c)r, contained in A (3.17.11), and we are back to the preceding case.

15 ISOLATED SINGULAR POINTS; POLES; ZEROS; RESIDUES

241

15. I S O L A T E D S I N G U L A R P O I N T S : POLES; ZEROS; RESIDUES

(9.1 5.1) Let A be an open subset of C, a mi isolatedpoint of C - A (3.1 0.1 0), r a number > O such that all points of the ball Iz - a1 < r except a belong to A. I f f is an analytic mapping of A into a complex Banach space E, then f o r 0 < Iz - a1 < r, we have f(z)=

2 cn(z - a)" + c dn(Z - a>-, m

a:

n=O

n= 1

where both series are convergent f o r 0 < Iz - a1 < r, and (X

tvliere y is the circuit t -+ a

+ re"

(0 < t

- u ) " - ~ ~ ( dx, x)

< 2n).

This follows at once from (9.14.2) applied to the ring p where p is arbitrarily small. Observe that the series u ( x ) =

< ( z - a ( < r,

C d n x " is an entire function

such that

n= I

u(0) = 0; we say that the function u(l/(z - a)) is the singular part o f f in the neighborhood of a (or at a). When u = 0, f coincides in the open

set U : 0 < Iz - a / < r with the function g ( z ) =

a:

c,(z - a)", which is

n=O

analytic for Iz - a1 < r ; conversely, iff is the restriction to U of an analytic function fi defined for Iz - a1 < r, then f i = g by (9.9.4) and (9.15.1), hence u = 0. When u # 0, we say that a is an isolated singular point off. If u is a polynomial of degree n 2 1, we say a is a pole of order n o f f ; if not (i.e. if d,,, # 0 for an infinite number of values of m) we say a is an essential singular point (or essential singularity) of j : In general, we define the order w(a; f ) or w(a) off at the point a as follows: o(a) = - co if a is an essential singularity; w(a) = -n if a is a pole of order n 2 1 ; w(a) = rn iff # 0, u = 0 -"

m

and in the power series

1 c,(z - a)"

equal to f ( z ) for 0 < Iz - a1 < r,

n=O

m is the smallest integer for which c,, # 0; finally w(a; 0) = +a.When w ( a ;f ) = m > 0 , we also say a is a zero of order m off. Observe that if both A g are analytic in the open set U : 0 < Iz - a1 < r, and take their values in the same space, then w ( a ; f + g ) 2 min(w(a; f ) , w(a; g ) ) ; if one of the functions f, g is complex valued, then w(a;f g ) = w(a;f ) w ( a ; g ) when one of the numbers w ( a ;f ) , w ( a ; g ) is finite. Any function f analytic in U and of finite order n (positive or negative) can be written in a unique

+

242

IX ANALYTIC FUNCTIONS

way ( z - a)yl, where fl is analytic in U and of order 0 at the point a. Finally, iffis analytic in U and complex valued, and of finite order n7, then it follows from the principle of isolated zeros and from (9.3.2) that there exists a number r' such that 0 < r' < r and that I/f is analytic in the open set 0 < Iz - a1 < r ' ; we have then w(a; IF) = - w(a;f). (9.15.2) Let f be analytic in the open set U : 0 < Iz - a1 < r . In order that o ( a ;f ) 2 n where n is a positive or negative integer, it is necessary and sufficient that there exist a neighborhood V of a in C such that ( z - a)-"f ( z ) be bounded in V n U. The condition is obviously necessary, since a function having order 3 0 at a is the restriction of a function analytic in a ball Iz - a1 < r. Conversely, by considering the function ( z - a)-"f(z), we can suppose n = 0. Then it follows from (9.15.1) and the mean value theorem that if 11 f(z)II d M in U , we have, for any p such that 0 < p < r, lld,,t,ll d Mp" for any m 3 1 ; as p is arbitrary, this implies d,,, = 0 for each m 2 1. Q.E.D. The coefficient d, in (9.15.1) is called the residue off at the point a.

PROBLEMS

1. Show that there are no isolated singular points for analytic functions of p > 2 complex variables (in other words, if A is an open subset of C p ,a E A and a mappingfof A - { a } into a complex Banach space E is analytic, it is the restriction of an analytic mapping of A into E; use Problem 5 in Section 9.10). 2. Letfbe a complex valued analytic function of one complex variable having an essential singularity at a point a E C; show that for any complex number h, it is impossible that the function l / ( f - h ) should be defined and bounded in an open set of the form V - {a},where V is an open neighborhood (use (9.15.2)). Conclude that for any neighborhood V of a such that f i s analytic in V - {a},f(V - { a ) ) is dense in C (" Weierstrass' theorem"; see Section 10.3, Problem 8). 3. An entire function which is not a polynomial is called a trunscendental entire function. Let f be a complex valued entire transcendenial function of one complex variable. (a) Show that for any integer n > 0, the open subset D(n) of C consisting of the points z E C such that If(z)i > n is not empty and cannot contain the exterior of any ball (apply Problem 2 to the function f(l/z)). (b) Let K(n) be a connected component (3.19.5) of D(n). Show that K(n) is not bounded and that If(z)i is not bounded in K ( n ) (if u $ K(n), consider the function f(l/(z - a ) ) and use Problem 14 of Section 9.5). (c) Show that there is a continuous mapping y of [0, K>[into C, such that in every interval [0, a ] ,y is the primitive of a regulated function, and that lim iy(t)i = +a

+

r-ti

IU

1 5 ISOLATED SINGULAR POINTS; POLES; ZEROS; RESIDUES

and lim If(y(t))l r-+m

=

243

+a.(Consider a sequence of open subsets L, c C such that L, is

a connected component of D(n), and L.+l C L, for every n ; the existence of such a sequence follows from (b). Use then (9.7.2)) (d) Extend the preceding results to complex valued entire transcendental functions of an arbitrary number of complex variables. (If f ( z I , . . . , z), = a n l . . “,zl n l . . . z p , there exists at least an index k such that there are infinitely many monomials with non zero coefficient R , ~ ,. . and arbitrarily large n k . On the other hand, prove that if (9,) is a denumerable family of entire complex valued functions of p complex variables, none of which is identically 0, then there exist points (cl, . . . , c,) for which g,,(cl,.. . , c,) # 0 for every m ; to do this, use induction on p , and the fact that for a function h(z) of one complex variable, analytic in A c C and not identically 0, the set of solutions z of h(z) = 0 is at most denumerable (see (9.1.5)).) 4. Let ~ ( xbe) an arbitrary increasing and positive real function defined in [0, cc [. Let (k.) be a strictly increasing sequence of integers such that k1 = 1 , and (n(n - 1))”“ > ~ ( n1 ) for n > 1. Show that the power series

+

f(z)= 1

+

+ 5 (L)k” n- I n=2

is convergent for all z E C, and that for every real x > 2, f ( x ) > ~ ( x(in ) other words, there are entire functions which tend to infinity “faster” than any given real function). 5. For any real numbers a , p such that p > 0, let L=J be the endless road (Section 9.12, Problem 3) defined as follows: for t < - I , L,,p(t) = a - ip - t - 1 ; for - 1 < t < I , L&) = a ipr; for f > 1, L,,&) = a ip r - 1. Let G.,p = LnSp(R). (a) Show that if 77/2 < p < 3rr/2, and if x $ Ga,pthe function z (exp(exp z))/(z - x) isimproperlyintegrablealongL..B. Furthermore,isP1,P2aresuchthat I-”(x)l < < p2 of /4(x)1> p2 > or a ( x ) < a, the integrals along and Llr.ozare the same; similarly, if 9 ( x ) < a1< a 2 ,or g1 < cc2 < B(x), or [.f(x)( > p the integrals along and La2,Aare the same (use Cauchy’s theorem). (b) Deduce from (a) that if L = Lo,11,

+ +

+

--f

PI,

PI

can be extended to an entire function. (c) Show that

(prove that the integral along La,” of exp(exp z ) is independent of cc and p (provided n-/2 < p < 3n-12)). (d) Show that if x belongs to the open set A defined by W ( x ) < 0 or i-”(x)l > rr,

where F(:) is bounded in A (express F(x) by an integral along Lo,pwith p < rr, using (a) and (c)). (e) Show that if x belongs to the open set B defined by W ( x )> 0 and IY(x)l < rr, then 1

E(x) = exp(exp x ) - x

GW +x2

IX ANALYTIC FUNCTIONS

244

where G(x) is bounded in B (prove first, using Cauchy's formula, that if and /9(x)1< x , then

-1

< 9 ( x )< 0

where L'= L - l , n . Show next that that formula is still valid for x E B using (9.4.2) and express G ( x )by an integral along L - l . f lwith p > x ) . (f) Let H(x) = E(x)e-"("'; show that H is an entire transcendental function such that lim H(rejB)= O for every real # (use (d) and (e); compare with the result of v-+m

Problem 3). 6. Let f be a complex valued entire function of p > 2 complex variables. Show that if f ( a l ,..., a,) = b, then for every r > 0 , there exists z = (zI. ..., z,) such that Izp - ukI2= r 2 and f ( z I , . . . ,z,) = b (use Problem 5(b) of Section 9.10). k

7. Letfbe an analytic mapping of an open subset A c C p into a complex Banach space E. A frontier point zo of A is called a regular point forfif there is an open neighborhood V of zo and an analytic mapping of A u V into E which coincides with f i n A. A frontier point of A is said to be singular forfif it is not regular. (a) Let R < +a, be the radius of convergence (Section 9.1, Problem I ) of a power series f ( z ) =C a, z" of one complex variable. There is at least one point zo such that

lzol = R which is a singular point for f. (Otherwise one could cover the circle Iz/ R with a finite number of open balls Bk whose centers hk are on that circle, and such that in each open set B(0; R) u Bx there is an analytic function 6 coinciding with f in B(0; R). Show that for any two indices h, k for which Bh n Bx # @,fh andfx coincide in Bh n Bk, using (9.4.2), and conclude from (9.9.1) and (9.9.2) that the radius of convergence of CI, z" would be > R.)

"

(b) With the notations of (a), suppose a, 0 for every n. Show that the point z = R is singular for f. (One may suppose R = I . Let el" be a singular point for f; then for 0 < I' < I, the radius of convergence of the power series ~ f ( " ' ( r e ' Q ) z " /is t i !exactly 1 - r (9.9.1). Observe that If(")(re")l < f ( " ) ( r ) ,and use (9.1.2).) (c) With the notations of (a), suppose R = I . Let b, c be two real numbers such that 0 < h < 1, c 1 - h, and let p be an integer > I . In order that the point z = 1 be a singular point for f, it is necessary and sufficient that the Taylor series 1 y'")(O)u"/ir! ~

"

+

for the function y(u) = f(bu, c u p + ' )have a radius of convergence equal to I . (Observe that if I { I ~ < I , Ibu" c u P f 1 I < 1, and that the two sides of the last inequality can only be equal for u = 1. The proof for the necessity of the condition has to use (10.2.5), in order to show that there is in the neighborhood of z = I an analytic function h(z) such that z = g(h(z)) in that neighborhood.) (d) Suppose (with the notations of (a)) that a, = 0 except for a subsequence ( t i x ) of integers such that nk+l > (1 8)nk for every k , where 6 > 0 is a fixed number. Show that euery point zo on the circle IzI = R is a singular point for f(" Hadarnard's gap theorem "; the circle /zI = R is called a mrlmlboundary forf). (One may suppose

+

+

R = I . Use criterion of (c), taking p > l/8, and let g(u) =

m

C dnu" be

the Taylor

"=O

development of g at u = 0. By assumption, for given E > 0 there is a subsequence (mi) of integers such that llun,,~~ 2 (I- E ) ' " ~ (Section 9.1, Problem I). On the other hand the function F(u) (bup cu,+l)"" =C e.u" has I I = 1 as a singular point, by (b),

=c J

+

"

hence there is a subsequence (4,)of integers such that jeqll > (1 - ~ ) " l . l:dq~ll 2 (1 - E ) 2 4 9 .

Prove that

16 THE THEOREM OF RESIDUES

245

16. T H E T H E O R E M OF RESIDUES

We first recall that any subset S c C the points of which are all isolated is at most denumerable, for the subspace S of C is then discrete and separable (by (3.9.2), (3.20.16), and (3.10.9)), hence S is the only dense subset of S (3 .I 0.1 0).

(9.16.1) Let A c C be a simply connected domain, (a,,) a (jinite or injinite) sequence of distinct isolated points of A, S the set of points of that sequence. Let f be an analytic mapping of A - S into a complex Banach space E, and let y be a circuit in A - S. Then I c v have

where R(a,,) is the residue off at the point a,, and there are only aJinite number of terms # O on the right-hand side (" theorem of residues"). We can obviously suppose each a, is a singular point for f, for we can extend f by continuity to all nonsingular points a,, which does not change both sides of the formula (since R(a,) = 0 if a, is not singular). Under that assumption, for any compact set L c A, L n S is jinite, for L n S is closed in L, as A - S is open in C by definition; hence L n S, being compact and discrete, is finite (3.16.3). Let I be the interval in which y is defined, and let P be the set of points x E A such that j ( x ; y) # 0. We know (9.8.6) that the closure P of P in C is compact, and P does not contain any frontier point of A, for such a point cannot be in y(I), nor have index # O with respect to y , by (9.8.7); as the set of points x in C - y(1) where the index ,j(x;y) takes a given value is open (9.8.3), any point in P which does not belong to y(1) is in P, hence P c A. On the other hand, let q(t, 5 ) be a loop homotopy in A of y into a one-point circuit ( t E I, ( E J, where J is a compact interval). Then M = cp(I x J) is a compact subset of A. Let H c N be the finite set of the integers n such that a,€ M u P; for each n E H, let zr,(l/(z - a,,)) be the singular part off at the point a,. Let B be the complement in A of the set of points a, such that n 4 H ; then B is open, for a compact neighborhood of a point of B, contained in A, has a finite intersection with S. By definition of the singular parts, there is a function g, analytic in B, and which is equal to f ( z ) -

at every point z # a,, ( n E H).

246

IX ANALYTIC FUNCTIONS

As M c B by definition, y is homotopic in B to a one-point circuit; hence, by Cauchy’s theorem, g(z) dz = 0, in other words

si

the result then follows from (9.14.2), applied to each of the functions u,, in an open

“

ring ” of center a,, , containing y(1).

17. M E R O M O R P H I C F U N C T I O N S

Let A be an open subset of C, S a subset of A, all points of which are isolated. A mapping f of A - S into a complex Banach space E is said (by abuse of language) to be meromorphic in A if it is analytic in A - S and has order > -co at each point of S. By abuse of language, we will always identifyfto its extension by continuity to all points of S which are not poles off; the argument used in (9.16.1) then shows that we can always suppose that for any compact subset L of A, L n S is jinite. Iff, g are two meromorphic functions in A, taking their values in the same space, and %whosesets of poles in A are respectively S, S‘, then S u S’ has all its points isolated, due to the preceding remark; f + g is defined and analytic in A - (S u S’), and has order > --GO at each point of S u S‘, hence is meromorphic in A (note that some points of S u S’ may fail to be singular for f + g). Similarly if f and g are meromorphic in A, and g is complex valued, f g is meromorphic i n A. Iff is meromorphic in A, S is the set of its poles, T the set of its zeros, then all the points of S u T are isolated; for if a E A and w(a) =h, then f ( z ) = ( z - a)hfl(z) in 2 set 0 < Iz - a1 < r, where A is analytic in ( z - a ( < r and f i ( a ) # 0; the principle of isolated zeros (9.1.5) shows that there is a number r‘ such that 0 < r‘ < r, and f ( z ) # 0 for 0 < Iz - a1 < r’. This proves our assertion, and shows moreover that L n ( S u T) is finite for any compact subset L of A (same argument as in (9.16.1)). In particular, iffis complex valued, l/fis meromorphic in A, S is the set of its zeros and T the set of its poles. Moreover, with the same notation as above, we have f ’ ( z ) = h(z - ~ ) ~ - l f , ( z () z - a)hf;(z) for 0 < Iz - a1 < r, hence f‘/f, which is analytic for 0 < I z - a1 < r‘, has order 0 at the point a if h = 0, order - 1 and residue h at the point a if h # 0.

+

(9.17.1) Let A be a simply connected domain in C , f a complex valued meromorphic function in A , S (resp. T) the set of its poles (resp. zeros), g an analytic function in A . Then, for any circuit y in A - ( S u T ) , we have

17 MEROMORPHIC FUNCTIONS

247

a$nite number of terms only being # 0 on the right-hand side.

This follows at once from the theorem of residues, for the residue of gf ’If at a point a E S u T is the product of g(a) by the residue off’/ at the point a. (9.17.2) W i t h the assumptions of (9.17.1) let t --t y ( t ) ( t E 1) be a circuit in A - ( S u T). Zfr is the circuit t +f ( y ( t ) ) ,then

For it follows at once from (8.7.4) that

hence the result is a particular case of (9.17.1) for g = 1. (9.17.3) (Rouchk’s theorem) Let A c C be a simply connected domain, f , g two analytic complex valued functions in A. Let T be the (at most denumerable) set of zeros o f f , T’ the set of zeros o f f + g in A, y a circuit in A - T, dejned in an interval 1. Then, if Ig(z)l < 1 f ( z ) l in y(I), the function f g has no zeros on y(l), and

+

+

The first point is obvious, since f ( z ) g(z) = 0 implies If ( z ) l = Ig(z)l. The function h = ( f + g)/f is defined in A - T and meromorphic in A ; we have

(-f + s > ’- -f’ h’ f+s f+h

in A -(T u T’).

Using (9.17.2), all we have to prove is that the index of 0 with respect to the circuit r : t+h(y(t)) is 0. As gif is continuous and finite in the compact set y(I), it follows from (3.17.1 0) and the assumption that r = sup Ig(z)/f(z)I < 1. In other words, is in the ball Iz - 1 I < r, and as SY(1)

0 is exterior to the ball, the result follows from (9.8.5).

248

IX ANALYTIC FUNCTIONS

(9.17.4) (Continuity of the roots of an equation as a function of parameters) Let A be an open set in C,F a metric space, f a continuous complex valued function in A x F, such that for each a E F, z +f ( z , a) is analytic in A. Let B be an open subset of A, whose closure B in C is compact and contained in A, arid let E F be such that no zero o f f ( z , ao) is on the frontier of B. Then there exists a neighborhood W of a. in F such that: (I) for any a E W, f ( z , a ) has no zeros on the froritier of B ; (2) for any a E W, the sum of the orders of the zeros o f f ( z , a ) belonging to B is independent of a. The number of distinct zeros of f ( z , ao) in B is finite; let a,, . . . , a, be these points. For each frontier point x of B, there is a compact neighborhood U, of x , contained in A, such that f ( z , ao) has no zero in U, (9.1.5); if we cover the (compact) frontier of B by a finite number of sets U x j , the union U of 8 and the U,, is a compact neighborhood of B, contained in A and such that f ( z , ao) has no zero in U n (A - 8).Let r be the minimum of the numbers la, - ajl (i # j ) , and for each i (1 < i < n), let Di be an open ball Iz - ail < ri of radius r, < r/2, contained in B ; then D i n Dj= @ if i # j . Let H =U Di); this is a compact set; let m be the minimum value of I f ( z , ao)l in H ; we have m > 0 by (3.17.10). Now, for each X E 8, there is a neighborhood V, of x contained in A and a neighborhood W, of a. in F, such that I f ( y , a)- f ( x , a,)l < m/2 for y E V, and a E W,. As 8 is compact it can be covered by a finite number of sets VXk (1 < k G p ) ; let W = WXk;this is a neighborhood of a. in F, and by definition, for any

-(u

k

W and any y E 8, we have If ( y , a ) -f ( y , ao)l < m. As a first consequence, it follows that f ( y , a) # 0 for y E H and a E W ; on the other hand, as I f ( z , a ) - f ( z , a0)l < If ( z , ao)l in H, RouchC’s theorem, applied to each circuit t + a, rieit (0 < t G27r) shows that the sum of the orders of the zeros off (z, a ) in Di is independent of a E W, hence the theorem. CY

E

+

PROBLEMS

1. Let A c C be an open simply connected set,fa nieroniorphic complex valued function in A, such that each pole off is simple and the residue of f a t each of these poles is a positive or negative integer. Show that there is in A a meroinorphic function g such thatf= g’/g. (If zo is not a pole off, show that for any point z1 E A which is not a pole off, and any road y in A, defined in I c R,of origin zo and extremity zl, and such that y(1) does not contain any pole off, the number exp 2.

0)

v)dx onlydependsonzoandzl,

and not on the road y satisfying the preceding conditions (use the theorem of residues).) Let f be an entire function of one complex variable, such that for real x , y , llf(x i-iy)ll < e l y 1 .Show that, for any z distinct from integral multiples nn of n,

17 MEROMORPHIC FUNCTIONS

249

(- 1)"f(nn)

+

(z

-m

-

m)'

where the series on the right-hand side is normally convergent in any compact subset of C which does not contain any of the points nn (n integer). (Consider the integral

's f(4 2ni

Yn

dx -sin x ( x - z)'

+

where yn is the circuit t + (n $ ) r e " for -n < t < n.Observe that for every E > 0, there is a number C ( E ) > 0 such that the relations Iz - n7r 2 E for every integer n E Z imply lsin zI > c(c)el,F(z)l;and use the theorem of residues.) 3. (a) Show that for z # nn (n integer) 1 cotz=-+C z

22 z2-n2nz ~

"=I

where the right-hand side is normally convergent in any compact subset of C which does not contain any of the points nn. (Use Problem 2 and the relation lim (cot z - l/z) = 0.) I-0

(b) Deduce from (a) that

, the convergence being

where the product is defined as the limit of k=l

uniform f(z)

in

=n(1 +

01)

-m

every

4

--

compact

subset

of

C.

(Consider

the

entire

function

e - z / n n (Section 9.12, Problem l), and use (a) to prove that the

function (sin z ) / z f ( z ) is a constant.) (c) Deduce from (b) the identity

4.

(see Section 9.12, Problem 2). Let f be a complex valued function analytic in an open neighborhood A of 0 in C p ; for convenience we write w instead of zp and z instead of ( z l , . . . , z ~ - ~Suppose ). that f ( 0 , O ) = 0 and that the function f ( 0 , w ) , which is analytic in a neighborhood of w = 0 in C , is not identically 0. Then there exist an integer r > 0, r functions h,(z) analytic in a neighborhood of 0 in C P - l ,and a function g(z, w ) analytic in a neighborhood B c A of 0 in C p and # O in that neighborhood, such that f ( z , w ) = (w'

+ h,(z)w'-1 + .. . + h,(z))s(z, w )

in a neighborhood of 0 in Cp (the " Weierstrass preparation theorem"). (If f ( 0 , w) has a zero of order r at w = 0, use (9.17.4) to prove that there is a number E > 0 and a neighborhood V of 0 in C p - l such that for any Z E V, the function w 4 f ( z , w ) has exactly r zeros in the disc lwI < E and no zero on the circle Iwi =- E . Let y be the circuit t + w i t for -n < t < n;using the theorem of residues, show that there are

250

IX ANALYTIC FUNCTIONS

functions h,(z) (1 < j < r) analytic in V and such that the polynomial F(z, w) = wr+ hl(z)w'-' ... h,(z) satisfies the identity

+ +

for Z E V and IwI > E). Let (f.) be a sequence of complex valued analytic functions in a connected open subset A of C.Suppose that for each z E A, the sequence ( f . ( z ) )tends to a limit g(z), and the convergence is uniform in every compact subset of A. Suppose in addition that each mapping z -.f.(z) of A into C is injective. Show that either g is constant in A or g is injective (for any zo E A, consider the sequence (f.(z) -f.(zo)) and apply (9.17.4) and the principle of isolated zeros). Let be a real valued twice differentiable function in the interval 10,11. Suppose ) q#)cos x = 0 in 1-77, n [ . Iq~(0)l< Iv(l)l, and let xo be one of the zeros of ~ ( 0 Show that the entire function F(z) =

IO1

p(r) sin zt df

has a denumerable set of zeros; furthermore it is possible to define a surjective mapping n -.z, of Z onto the set of zeros of zF(z), such that each zero corresponds to a number of indices equal to its order, and that lim ( z ~ ~ - x "2n7r)=

+

n-fm

lim ( z ~ ~ xo+ -~2n7r) = 0. (Integrating twice by parts, show that one can

n-fm

+

zF(z) = ~ ( 0) p(1) cos z G ( z ) , where IG(z)l < u e l f l ( z ) l / l z / ;minorize j ~ ( 0) ~ ( 1 )cos zI outside of circles having centers at the zeros of that function, in

write

the same was as lsinzl was minorized in Problem 2 ; and use RouchC's theorem in a suitable way.) Treat similarly the cases in which I ~ ( 0 )> l Ip(1)l or ly(0)l = /q~(l)j.

APPENDIX TO CHAPTER IX

APPLICATION OF ANALYTIC FUNCTIONS TO PLANE TOPOLOGY (Eilenberg’s Method)

1. I N D E X OF A P O I N T WITH RESPECT TO A L O O P

(Ap.1.1) If t + y(t) (a < t < b) is a path in an open subset A of C , there is in A a homotopy cp of y into a road yl, such that cp is defined in [a, b] x [0, 13 and ~ ( a5), = y(a) and q(b, t) = y(b)for every 5 E [0, I ] .

Let I = [a, b ] ;as y(1) is compact, d(y(I), C - A) = p is > 0 (3.17.11). Asy is uniformly continuous in I (3.16.5), there is a strictly increasing sequence (tk)06ksmof points of I such that to = a, t, = b, and the oscillation (Section 3.14) of y in each of the intervals [tk, t,,+l] (0 f k f m - 1) is < p. Define (0 < k < m - 1); it is clear that y1 is a road, with yl(a) = y(a), y,(b) = y(b), and yl(I) is contained in A, since y l ( [ t k , tk+,])is contained in the open ball of center Y(tk) and radius p. Define then cp(t, t) = ty,(t) (1 - <)y(t);it is readily verified that cp(t, 5) is in the open ball of center y(tk) and radius p for tk < t f tk+l and 0 < 5 < 1 (0 f k < m - 1); hence cp verifies the required conditions.

+

In particular, if y is a loop, we see that cp is a loop homotopy in A of y into a circuit yl. Consider now any loop y in C, defined in I, and any point a $ y(1). As there are, by (Ap.I.I), circuits y1 which are homotopic to y in C - { a } , we can define the index j ( a ; y ) as equal to j ( a ; yl) for any circuit homotopic to y in C - { a } ; by Cauchy’s theorem (9.6.3), this is independent of the particular circuit y1 homotopic to y in C - {a}. 251

252

APPENDIX TO CHAPTER IX

Using (Ap.1 .I) it is readily verified that the index of a point with respect to a loop does not depend o n the origin of the loop (Section 9.6), and that properties (9.8.3), (9.8.5), (9.8.6), and (9.8.7) still hold when "circuit" is replaced by " loop" in their formulation.

2. ESSENTIAL MAPPINGS IN T H E UNIT CIRCLE

Let E be a metric space. We say that a continuous m a p p i n g f o f E into the unit circle U: IzI = 1 is itiessential if there is a continuous mapping g of E into R such that f ( x ) = eiq("' for every x E E. A continuous mappingf of E into U is called essential if it is not inessential.

lf,f,, f 2 are inessential mappings of E into U, flf 2 and l/fl =jlu r e inessential; iff i is essential and f 2 inessential, then f l f 2 and f i r 2 are essential.

(Ap.2.1)

(Ap.2.2) I f f is an inessential mapping OfE into U, andg a continuous niapping of a metric space F into E, then f 0 g is inessential.

These properties are obvious consequences of the definition.

(Ap.2.3) Any continuous niapping f of a metric space E into U such tliat f(E) # U is inessential.

Let toE U -f(E). There is a E R such that to = e'", and the restriction of t -+ eit to the open interval ]a, a 2n[ is a homeomorphism of that interval onto U - { l o }(9.5.7); if $ is the inverse homeomorphism, we have f ( x ) = e i $ ( f ( x )for ) every x E E. Q.E.D.

+

(Ap.2.4) If',fi,f 2 are trrso continuous mappings of a metric space E into U , such that f l ( x ) # -f 2 ( x )for any x E E, and iff; is essential (resp. inessential), so is f2.

For f =fl/fi is a continuous mapping of E into U which does not take the value - 1, hence is inessential by (Ap.2.3).

2

ESSENTIAL MAPPINGS IN THE UNITCYCLE

253

(Ap.2.5) Let E be a compact metric space, 1 = [O, 11, f a continuous mapping of E x I into U. If the mapping x +f(x, 0 ) is essential (resp. inessential), so is the mapping x - + f ( x , 1).

As f is uniformly continuous in E x I (3.16.5), there is an integer n 2 1 such that the relation 1s - t I < l / n implies I f ( x , s) -f (x, t)l < 1 for any x E E. Let ,fk(x) = f ( x , kin) for 0 6 k < n ; we therefore have Ifk(x) - f k + l ( x ) I 6 1 for any x E E and 0 < k < n - I , and as Ifk(x)l = Ifk+l(x)I = 1 for x E E, we havef,(x) # - f k + l ( x ) for x E E. Hence the result by (Ap.2.4).

Any continuous mappirig f of a closed ball (in R”) into U is inessent i d .

(Ap.2.6)

Let E be the ball d(x, a ) < r, and define g(x, t ) =f ( a + t(x - a ) ) ; then g is continuous in E x [O, 11, g(x, 1) =f ( x ) and g ( x , 0) =f ( a ) ; as x -+ g(x, 0) is inessential (Ap.2.3), so isf by (Ap.2.5).

Let A , B be tityo closed subsets of a metric space E, such that E = A u B, and that A n B be connected. Let f be a continuous mapping of E into U; if the restrictions o f f to A and B are inessential, f is inessential.

(Ap.2.7)

There are, by assumption, continuous mappings g, h of A and B into R such that f (x) = eiY(’) in A, f ( x ) = eih(’) in B. For x E A n B, we have therefore eiq(” = eih(x),hence (9.5.5) (g(x) - h(x))/2n is an integer; but as g - h is continuous in the connected set A n B, this implies g - h is a constnnt 2nn in A n B by (3.19.7). Let then u(x) = g ( x ) in A, u(x) = h(x) 2nn in B; it is clear that f ( x ) = eiu(x)in E, and as g and h + 2nn coincide in A n B, u is continuous in E. Q.E.D.

+

In order that a continuous mapping f of U into itself be essential, a necessary atidsuflcient condition is that j ( 0 ; y) # 0 f o r the loop y : t f(e’‘) (0 < t < 2n).

(Ap.2.8)

-+

By (9.8.1) we can write f ( e ” ) = eiS(‘),where $ is continuous in [0, 2n], and $(2n) - $(O) = 2nn by (9.5.5), n being the index j ( 0 ; y). Let = eir (0 < t < 2n) and for w(t, 5 ) = $ ( t ) + t ( n t -t- $(O) - $ ( t ) ) ; if, for O < 5 < I , we write g(i, 5 ) = eiu(‘,c’, g is continuous in (U - {I}) x [0, 11 by (9.5.7), and as e ; 1 J ( 2 n * < = ) ei”(0.C’ - f(1) for any 5 , g is extended by

254

APPENDIX TO CHAPTER IX

continuity to U x [0, 11. By (Ap.2.5), we are thus reduced to proving the theorem for the mapping f :[ [". It is clear that for n = 0, f is inessential. Suppose n # 0, and let us prove by contradiction that f cannot be inessential. Otherwise, there would exist a nonconstant continuous mapping It of U into R such that ["= e i h ( ( in ) U. As h(U) is a compact (3.17.9) and connected ((9.5.8) and (3.19.7)) subset of R, h(U) is a compact interval [a, b] with a < b (3.19.1). Let lo E U such that /I([,) = a . We therefore have [: = cia; there is a neighborhood V of lo i n U such that the oscillation (Section 3.14) of h in V be < n ; on the other hand, (9.5.7) applied to the interval ]a - n/n, a + n/n[, proves that there exists a point [ E V such that [ n = e i ( a - E ) , where E > 0 is sufficiently small. By (9.5.5), h([) - (a - E ) is a multiple of 2n, and the choice of V implies that this multiple can only be 0 as soon as E n; but this contradicts the definition of a. --f

(Ap.2.9)

The identity mapping [ 4 [ of U onto itself is essential.

3. CUTS OF THE PLANE

In a metric space E, we say a subset A of E separates two points x , y of E - A if the connected components (Section 3.19) of x and y in E - A are distinct. We say that A cuts E (or is a cut of E) if E - A is not connected. For any two points a, b of C such that a # b, let s a , h ( Z ) be the function z + ( z - a)/(z - b), defined in C - { b } ; it is readily verified that so, b is a homeomorphism of C - {b} onto C - {I}. (Ap.3.1) (Eilenberg's criterion) Let H be a compact subset qf C; in order that H separate two distinct points a, b of C - H, a necessary and sufjkient condition is that the mapping z 4 s a , ,,(z)/ls,, &) I ofH into U be essential.

(a) SufJiciency. Suppose a and b are in the same connected component A of C - H. As C - H is open in C and C is locally connected ((3.19.1) and (3.20.16)), A is open in C (3.19.5). By (9.7.2) there is a path t + y ( t ) in A, defined in I = [0, I ] , such that y(0) = a, ?(I) = b. As y ( t ) 4 H for any value of t , the mapping ( z , t ) + f ( z , t ) = s a , ?(,)(z)/1sa, y ( t ) ( ~ ) I is continuous in H x I, and f ( z , 0) = 1, f ( z , I ) = sa, b(z)/Is,, b ( Z ) ( ; the result follows from (Ap.2.5). (b) Necessity. Let A be the connected component of C - H which contains a ; A is open in C and all its frontier points are in H (they cannot

4 SIMPLE ARCS A N D SIMPLE CLOSED CURVES

255

be in another connected component of C - H, otherwise A would have common points with that open component (Section 3.8)); therefore A u H is closed in C,and as b $ A u H, we have d(b, A u H) > 0. Let A‘, H’ be the images of A, H under the homeomorphism Z - + S , , ~ ( Z ) of C - {b} onto C - { 1) ; H’ is compact and A‘ is a connected open subset of C - H’, which is bounded and contains 0. Moreover, the frontier points of A‘ in C are points of H’ and (possibly) 1 ; hence A’ is compact and so is A’ u H’. In addition, if 1 belongs to the boundary of A’, this means that A is unbounded, hence has points in common with the exterior of a ball containing H ; but as that exterior is connected (9.8.4), it is contained in A by definition of a connected component (Section 3.19). This shows that there is a ball V of center I , such that V - (1) c A’, hence 1 is not a frontier point of C - A‘, which proves that the frontier of C -A’ is always contained in H’. We have to show that the mapping u -+ u/lul of H’ into U is essential (Ap.2.2). Suppose the contrary; then there would exist a continuous mapping f of H’ into R such that u/lul = eif(”) for u E H’. By the Tietze-Urysohn theorem (4.5.1), f can be extended to a continuous mapping g of A’ u H’ into R. Define a mapping h of C into U by taking h(u) = u/lul for u E C -A‘, h(u) = eig(”)for u E A’; it follows at once from the definition of g that h is continuous in C. Let r > 0 be such that A‘ is contained in the ball B : IzI < r ; the restriction of h to B is inessential (Ap.2.6), and so is therefore the restriction of h to S : IzI = r. But the identity mapping [ -+ [ of U onto itself can be written h, gl,where h, is the mapping z -+ z/lzl of S onto U, and g1 the mapping -+ rc of U onto S. However, h, is the restriction of h to S, hence inessential, and therefore h, 0 g1 would be inessential (Ap.2.2), contradicting (Ap.2.9).

c

0

(Janiszewski’s theorem) Let A, B be two compact subsets of C, a, b two distinct points of C - (A u B). If neither A nor B separates a and b, and if A n B is connected, then A u B does not separate a and b.

(Ap.3.2)

From the assumption and (Ap.3.1) it follows that the restrictions of b(z)/Is,, b(Z)I to A and B are inessential; by (Ap.2.7) the restriction of that mapping to A u B is also inessential, hence the conclusion by (Ap.3.1).

z -+ s,,

4. SIMPLE ARCS A N D SIMPLE C L O S E D C U R V E S

An injective path t -+ y(t) in C, defined in I = [a, 81, is also called a simple path; a subset of C is called a simple arc if it is the set of points y(I) of a simple path. A loop y defined in I is called a simple loop if y(s) # y ( t )

256

APPENDIX TO CHAPTER IX

for any pair of distinct points (s, t ) of I, one of which is not an extremity of 1. A subset of C is called a simple closed curue if it is the set of points of a simple loop. Equivalent definitions are that a simple arc is a subset homeomorphic to [0, 11, and a simple closed curve a subset homeomorphic to the unit circle U (9.5.7).

The cornplernerit in C of a simple arc is corinected (in other words, a simple arc does not cut the plane).

(Ap.4.1)

Let y be a simple path defined in I, and let f be the continuous mapping of y(1) onto I, inverse to y . Let a, b be two distinct points of C - ?(I). By (Ap.3.1), we have to prove that the restriction cp of z -+ sa, b ( ~ ) / I ~ , , &z)I to y(1) is inessential. But we can write cp = (cp y) o f ; the continuous mapping cp y of I into U is inessential (Ap.2.6), and so is therefore cp by (Ap.2.2). 0

0

(The Jordan curve theorem) Let H be a simple closed curue in C. Then : (a) C - H has exactly two connected components, one of which is bounded and the other unbounded. (b) The frontier of every connected component of C - H is H. (c) I f y is a simple loop dejned it7 I and such tl7at y(1) = H, then j ( x ; y) = 0 i f x is in the unbounded connected cotnponent of C - H, and j ( x ; y ) = f.1 i f x is in the bounded connected component of C - H . (Ap.4.2)

The proof is done in several steps.

(Ap.4.2.1) We first prove (b) izithocrt any assumption on the number of components of C - H. Let A be a connected component of C - H ; as C - H is open, we see as in (Ap.3.1) that the frontier of A is contained in H. Let z E H , and let f be a homeomorphism of U onto H ; let [ = eie E U be such that f ( [ ) = z. Let W be an arbitrary open neighborhood of z in C, V c W a closed ball of center z ; then there is a number w such that 0 < w < n and that f ' ( e " ) E V for 0 - w < t < 0 + w ; let J be the image of that interval by t + f ( e " ) ; then the complement L of J in H is the image by t +f'(eit) of the compact interval [0 + w - 27t, 0 - w ] (9.5.7), and is a simple arc by (9.5.7). It follows from (Ap.4.1) that the open set C - L 3 C - H is connected. Therefore (9.7.2) for any x E A c C - L, there is a path y in C - L, defined in I = [a, b ] , such that y(a) = x,y(b) = z.

4

SIMPLE ARCS A N D SIMPLE CLOSED CURVES

257

The set y(l) n J is compact and contained in V ; let M be its inverse image by y, which is a compact subset of I , such that a 4 M ; let c = inf M > a. Then the image by y of the interval [a, c[ is a connected set P ((3.19.7) and (3.19.1)), which does not meet J nor L, hence is contained in C - (J u L) = C - H ; as P contains x, it is contained in A by definition. But when t < c tends to c, y ( t ) E A tends to y(c) E V, hence y ( t ) E W as soon as c - t is small enough; this shows that z E A. Q.E.D. (Ap.4.2.2) We next prove the theorem under the additional assumption that H contains a segmtvit S with distinct extremities. Applying to C a homeomorphism z +Az I- ,u, we can suppose S in an interval [ - a , a ] of the real line R. Let p = d(0, H - S) d a, and consider an open ball D: IzI < r , with r < p ; then D n (C - H) = D n (C - S), and it is clear that D n (C - S) is the union of the two sets D,: IzI < r , 4 z > 0 and D,: Jz)< r , f z < 0, which have no common points. It is immediately verified that the segment joining two points of D, (resp. D2) is contained in D, (resp. DJ, hence (3.19.3) that D,, D, are connected. On the other hand, we have seen in (Ap.4.2.1) that every connected component of C - H meets D, hence meets D, or D.,; but if two connected components of C - H meet D, (resp. D2), they are necessarily identical, since D, (resp. D,) is connected and contained in C - H (3.19.4). This proves that C - H has at most t ~ v oconnected components. We prove next that C - H is not conriected, hence has exactly fbvo components. Suppose the contrary, and let x E D,, y E D,; as D is connected, C - D does not separate x and y ; on the other hand, if C - H is connected, H does not separate x and y . But H n (C - D) is the complement in H of the open interval 1-r, r [ ; by (9.5.7), this complement is therefore a simple arc, hence connected. By Janiszewski's theorem (Ap.3.2), the union H u (C - D) does not separate x and y ; but this is absurd, since the complement of H LJ (C - D) in C is D, u D,, and D,, D, are open sets without common points, hence D, LJ D, is not connected. As H is compact, it is contained in a ball of center 0, whose complement in C is connected, hence contained in a connected component of C - H ; this shows one of these components A is unbounded, and the other B is bounded. Moreover, it is clear that j(x;y) = 0 when x E A (9.8.5). On the other hand, D, is contained in one of the components of C - H, D, in the other; all we need to prove therefore is that j ( x l ; y) - j ( x 2 ;y) = 5 1 for one point x, E D, and one point x, E D, (9.8.3). Supposing the origin of y to be the point a E S, let J c I be the inverse image by y of H - S, which is a compact interval [s(, p] and let y, be the path t + y ( t ) defined in J, of extremities - a and a. By (Ap.1.1) there is a homotopy 'p, in C - D of

258

APPENDIX TO CHAPTER IX

y, into a road y 2 , such that cp, is defined in J x [0, I ] and cp,(cc, t) = y(cc), q l ( p , t) = y(P) for any 5. Define cp in I x [0, I ] as equal to cpl in J x [0, I ] and to y ( t ) for any ( t , 5 ) ~ ( -1 J) x [0, I ] ; then for any x1 E D , (resp. x2 E D2), cp is a loop homotopy in C - {x,} (resp. C - {x2})of y into a circuit y 3 . We can therefore limit ourselves to proving that j ( q ; y) - j ( x 2 ;y ) = +_ 1 when y is a circuit defined in I, having the following properties: (1) S c ?(I) and if T is the inverse image y - ’ ( S ) , then T is a subinterval of I and the restriction of y to T is a homeomorphism of T onto S; ( 2 ) y(I - T) is contained in C - D (note that perhaps this new y is not a simple loop). Then the inverse image by y of the interval [ - r , r ] is a subinterval [A, p ] of T ; suppose for instance that ?(A) = - r , y ( p ) = r . We can suppose (replacing y by an equivalent circuit) that I = - 7 1 , p = 0 , and moreover that - r is the origin of y, so that I = [ - 7 1 , w ] with w > 0. Take xl = i t , x2 = -it with 0 < 4 < r ; let cr be the road t + y ( t ) , - 7 1 d t < 0, 6, the road t -+ rei‘, --n b t b 0, 6, the road t -+ r e - i t , - n < t < 0. Then, Cauchy’s theorem applied in the halfplane 4 ( z ) < [ (resp. 9 ( z ) > - 4 ) which is a star-shaped domain (9.7.1) yields dz

dz

and

dz z

+ it‘

Hence

Now the left-hand side is independent of 5 , and when t tends to 0, the righthand side tends to 2ni, using the fact that I y ( t ) l > r for 0 ,< t b w, the mean value theorem (to majorize the last integral), and (8.11.1).

(Ap.4.2.3) We now turn to the case in which H contains no segment with distinct extremities. Let a, b be two distinct points of H, S the segment of extremities a, b ; we may again suppose that S is a closed interval in R. By assumption, there is at least one point x E S n (C - H); let J be the connected component of x in S n (C - H), which is an open interval ] y , z [ since S n (C - H) is open in R ((3.19.1) and (3.19.5)); moreover its extremities y , z are in H. Let g be a homeomorphism of H onto the unit circle U, and let g(y) = e”, g(z) = eid, where we may suppose that c < d < c 271 (9.5.7). Let U,, U, be the simple arcs, images of t -+ e i f , c < t b d , and t -+ eit, d d t < c + 27r, and let H,, H, be their images by the homeomorphism f of U onto H, inverse to g. Using (9.5.7), we see immediately that there is a

+

4 SIMPLE ARCS AND SIMPLE CLOSED CURVES

259

homeomorphismf, (resp.f,) of U, (resp. U,) onto the closed interval J = [ y , z ] , such that f,(eic) =f2(ei‘) = y , f i ( e i d )= f 2 ( e i d )= z. Let h, (resp. I?,) be the mapping of U into C, equal to f in U, (resp. in U,), tof, in U, (resp. to& in U,); the definition of J implies that h,, h, are homeomorphisms of U onto two simple closed curves GI = HI u J, G , = H, u J, each of which contains the segment J. Let w E H,, distinct of y and z ; there is an open ball D of center w, which does not meet the compact set G, . From (Ap.4.2.1), each connected component of C - G, has points in D; moreover, if id, wf’ are two points of D in a same connected component of C - GI, iv’ and M”’ are not separated by GI ; they are not separated either by G, , since they belong to D c C - G, which is connected. But GI n G, = J is connected, hence, by Janiszewski’s theorem (Ap.3.2), iv’ and W” are not separated by GI u G , , nor of course by H c G, u G, . In other words, id and w” belong to the same connected component of C - H. But as C - G, has exactly two connected components, and each connected component of C - H has points in D by (Ap.4.2.1), it follows that C - H has at most two connected components. On the other hand, it follows from (Ap.4.2.2) that there are two points NJ’, iv” in D which are separated by G,. We show they are separated by H. Otherwise, as they are not separated by G, , and G, n H = H, is connected, they would not be separated by G, u H 3 GI (Ap.3.2), contrary to assumption. We have thus shown that C - H has exactly two connected components; the same argument as in (Ap.4.2.2) proves that one of them, A, is unbounded and the other, B, is bounded. Finally, we can suppose y is the origin of the loop y, and, if I = [a, PI, that HI = y([a, A]), H, = y ( [ A , /I]). Define the loops y1 and y, as follows: yl(t) = ( t - a + l)(y - z ) + z for a - 1 < t < M, yl(t) = y ( r ) for a < t < A; y 2 ( t ) = y ( t ) for A < t < /I, y 2 ( t ) = y ( t - P)(z - y ) for P < t < P 1. Using (Ap.1 .I) it is immediately verified that for any point x # G, u G , , j ( x ; y ) = ,j(x; yl) + j(x; y,). With the same meaning as above for D, let again M“, w’’ be two points of D separated by GI; then we have j ( d ; y,) = j ( w ” ; y,) since iv’ and KJ“ are not separated by G, (9.8.3), and j ( s ’ ;y , ) - j ( w ” ; 7,) = k 1 by (Ap.4.2.2). From this it follows that j ( w ’ ; y) - ,j(it,”; y ) = k I , which ends the proof.

+

+

(Ap.4.3) Let H be a simple closed curve in C, D the bounded connected component of C - H. Then, for any loop y in D, j ( x ; y ) = 0 f o r any x E H.

Let U be an open ball of center x, having no common points with the set y(1) of points of y. There exists in U a point z E C - (D u H) = C - D (Ap.4.2), and as U is connected, j ( x ; y) = j ( z ; y ) (9.8.3). But j ( z ; y) = j ( y ; y)

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APPENDIX TO CHAPTER IX

for all points y of the unbounded connected component C - of H (9.8.3), and there are points y E C - r> which are exterior to a closed ball containing y(1); for such points, j ( y ; y) = 0 (9.8.5),hence the result.

PROBLEMS

1. Let A be a connected open subset of C; show that for any two points a , h of A, there is a simple path y contained in A , having a and b as extremities, and whose set of points is a broken line (Section 5.1, Problem 4; this amounts to saying that y is piecewise linear). (Use a similar argument as that in (9.7.2).If a “ square” Q = 1 x 1 c A (I closed interval with nonempty interior in R) is such that a @ Q, and there is a simple path t + y l ( t ) in A, defined in J c R, with origin a and extremity c E Q, consider the E Q, and observe that the segment of extremities smallest value f o E J such that rl(ro) y l ( t o )and any point of Q is contained in Q.) 2. Is Janiszewski’s theorem still true when A and B are only supposed to be closed subsets of C, even if A n B is compact (and connected)? Show that the statement of the theorem remains true in the two following cases: (1) A, B are two closed sets, one of which is compact; (2) A and B are two closed sets without a common point. (If c is a point sufficiently close to a , consider the mapping z+ l/(z - c), and the images of a, b, A and B under that mapping.) 3. For any simple closed curve H in C, denote by P(H) the bounded component of C H. (a) Let A be a connected open subset of C, H a simple closed curve contained in A. Show that A - H has exactly two connected components, which are the intersections of A and of the connected components of C - H (use Problem 2). (b) More generally, if H i (1 < i < r ) are r simple closed curves contained in A, and such that no two of them have common points, the complement of H i in A has ~

u

+

I

exactly r 1 components (use induction on r ) . (c) If H, H’ are two simple closed curves without a common point in C, show that or the closure of one of the sets P(H), P(H’) is contained in either P(H) n P(H’) = 0, the other. (Observe that if H c P(H’), the unbounded component of C - H’ has no common point with P(H), using (3.19.9).) (d) Suppose a connected open subset T of C has a frontier which is the union of r simple closed curves H, ( I < i < r ) , no two of which have common points. Show that there are only two possibilities: (1) T is unbounded and no two of the sets P(H,) have common points, their union being the complement of T; (2) there is one of the H , , say are contained in P(H,) for 1 < i < r - 1, no two of the -H , , such that the /$Hi) (1 < i < r - 1) have common points, and T is the complement of the union of the ,&H,) (for 1 < i < Y - 1) in P(H,). (If y , is a simple loop whose set of points is H i (1 < i < v ) , observe that the indices j ( x ; y , ) are constant for x E T, and that at most one of them may be # 0; otherwise, using (c), show that one at least of the H i would not be contained in the frontier of T.) 4. Let A be a bounded open connected subset of C, such that for any loop y in A and any z E C - A , j ( z ; y ) = 0. (a) Show that for any simple closed curve H c A, the bounded component P(H) is contained in A. (Observe that otherwise it would contain points of C - A, using (3.19.9) and part (b) of the Jordan curve theorem.)

B(H,)

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SIMPLE ARCS A N D SIMPLE CLOSED CURVES

261

(b) Let ( z 12’) be the euclidean scalar product xx’ 4 yy’ in the plane C = R2 (with z = x -t iy, z’ = x’ -t if). Let Po be the open hexagon defined by the relations

l(zl I ) I < 4,

l(zl eini3)1< 4,

I(zl e-ini3)l< 3.

For any number a > 0, the set of all hexagons aP,. deduced from aPo by all translations of the form me'"'^ I- n), where m and n are arbitrary integers in Z , is called an hexagonalnet of width a ; the sets aP,,,, (resp. aFn,,,)are called the open (resp. closed) meshes of the net; the boundary of aP,,,, is the union of 6 segments (the sides of UP,,), whose extremities are called the vertices of ctP,,,; the nodes of the hexagonal mesh are all the vertices of the meshes. Every node is a vertex of three meshes, and the other extremities of the three sides issuing from that node are called its neighboring nodes. Let B be the union of a finite number of closed meshes of an hexagonal net. Show that if a node belongs to fr(B), there are exactly two neighboring nodes which also belong to fr(B); conclude that fr(B) is the union of a finite number of disjoint simple closed curves, each of which is a union of sides of meshes of the net. (Start with two neighboring nodes a , , a , in fr(B), and show that one may define by induction a finite sequence (a,) of nodes belonging to fr(B), such that a, and a,,, are neighboring nodes for every n.) (c) Let B be the union of a finite number of closed meshes of an hexagonal net of width a , such that fr(B) is a simple closed curve (union of sides of meshes of the net). Let a , h be two neighboring nodes on fr(B); prove that there exists a continuous mapping (z,t ) --z ~ ( zt .) of B x [O, I] into B such that: ( I ) ~ ( z0). z in B; (2) ~ ( zt ,) = z for every t E [O, I ] and every z on the segment S of extremities a and b ; ( 3 ) ~ ( z1), c S for any z E B. (Use induction on the number N of meshes contained in B; consider one side of extremities c, d, contained in fr(B), such that c and d have the same first coordinate, equal to the supremum of pr,(B), and that d has the largest second coordinate among all the nodes in fr(B) having that supremum as first coordinate; show that if N > I , B = B, u P, where P is the unique mesh contained in B and having c and d as vertices, B, is the union of N 1 meshes, and has in common with P one, two or three sides; examine the various possibilities.) Prove that the interior of B is simply connected. (d) Let B be the union of all closed meshes of an hexagonal net of width a , which are contained in A; a is taken small enough for B to be nonempty. Let D be one of the (open) connected components of h ; show that fr(D) is a simple closed curve. (Use (a) and Problem 3(d), to prove that if fr(D) was the union of more than one simple closed curve, there would be simple loops y in A and points z E fr(A) such that j ( z ; y ) = 1 . ) (e) Conclude that A is simply connected, and is the union of an increasing sequence (D,) of open simply connected subsets, each of which is the bounded component of the complement of a simple closed curve (use (c) and (d)). Conversely, such a union is always simply connected. (f) Extend the result of (e) to arbitrary simply connected open subsets of C (for each n, consider the closed hexagons of the hexagonal net of width I / n which are contained in the intersection of A and of the ball B(0; n ) ) . (g) Let A be an open connected subset of C such that the complement C - A has no bounded component; show that A is simply connected (use (9.8.5)). (h) Prove that any connected component of the intersection of a finite number of open simply connected sets in C is simply connected. 5. Show that the following open subsets of C are simply connected but that their frontier is not a simple closed curve: (I) ThesetA, ofpointsx-I i y s u c h t h a t O < x < l , -2
262

APPENDIX TO CHAPTER IX

(In both cases, define an increasing sequence of bounded components P(H,,), where H, is a simple closed curve, such that the union of the /?(H,) is the given open set, To prove that fr(A,) is not homeomorphic to U, show that it is not locally connected, using (3.19.1) ; to prove the similar property for fr(A,) consider the complement of the point z = 1 in that set.) Are fr(A,) and fr(A2) homeomorphic? Let A be a simply connected open subset of C, distinct from C. Show that the frontier of A contains at least two distinct points. (Show that otherwise, one would have A = C - { a } , using (3.19.9) and the fact that C - { a } is connected to prove that there can be no exterior point of A ; conclude by using (9.8.4) and (9.8.7).) Lety be a simple loop defined in I = [0, 21, and let H = ~ ( 1 be ) thecorrespondingsimple closedcurve. Let abeasimplepathdefinedinIz= [1,2]andsuch t h a t : ( l ) a ( l ) = y ( l ) , 4 2 ) = y(2) = y(0); (2) a ( t ) E P(H) for every t E ] I , 2[; let L = ~ ( ( 1 2 ) . Define the simple loops y I , y 2in I by the conditions: Yl(t) = y(t)

for 0 s t < I ,

y l ( t ) = a@)

for

p ( t ) = a(2 - t )

for 0 < t < I ,

y z ( t )= y ( t )

for

< t < 2; I < t < 2.

1

Let HI = yl(l), HZ= y2(1). (a) Show that for any z E C which does not belong to H I u H z , j ( z ; y ) : A z ; yl) -i-j(z; y2) (use (Ap.1 .I)). (b) Prove that there are points zI E P(H) such that j ( z l ;y l ) = 0, and points z2 E P(H) such that j ( z 2 ;y2) = 0 (use (b) of the Jordan curve theorem (Ap.4.2)). (c) Deduce from (a) and (b) that P(H) is the union of /$HI), P(H2) and L n P(H), no two of these sets having common points. (a) ’ Let H be a simple closed curve, Lk ( I < k < n) n simple arcs, having their extremities in H, and whose points distinct from the extremities are in P(H). Suppose in addition that no two of the Lk have common points belonging to P(H). Then the interior of the complement of

u Lr in P(H) has n -t connected components, each n

k= I

1

of which is the bounded component of the complement of a simple closed curve in C. (Use induction on n, and Problem 7.) (b) Let H I , Hz be two simple closed curves in C , such that H I n Hz is finite. Show that each connected component of P ( H l ) n P(H2) is the bounded component of the complement in C of a simple closed curve (use (a)). Let y be a simple loop in C, defined in 1 = [- 1, I], let H = y(l), and suppose (for simplicity’s sake) that y(0) - 0 and the diameter of H is >2. Define inductively two decreasing sequences of numbers, (an)and (p,), tending to 0, such that pl I , a, is the largest number > O such that Iy(t)l < p. for (tl < an,and p n + I= inf

(. -; 1

8 . 3 9

where 6,, is the distance of 0 to the set of points y(r) such that (11 2 c(.. (a) Prove that if z, z’ are two points of P(H) such that IzI < P . + ~ and J z ’ <~ p n + , , then there is a path of extremities z, z‘, contained in the intersection of P(H) and of the closed disk of center 0 and radius p.. (Let L be a simple broken line of extremities z , z’, contained in P(H) (Problem I). Suppose first that the segment S of extremities z, z’ has no point in common with L, distinct from z, z ’ ; then R = L u S is a simple closed curve. Prove that if t E I is such that y(t) E P(R), then It1 < a,: observe that the intersection R n H is contained in S, and show that if there was a t E 1 such that y ( t ) E P(R) and (tl 2 a n ,there would be another t ’ I~such that y(t’) E S and It’l 2 a n , contradicting the definition of p,+,. Conclude in that case by taking the connected component of the intersection of P(R) and the open disk of center 0 and radius p,,

4 SIMPLE ARCS AND SIMPLE CLOSED CURVES

263

which contains points arbitrarily close to S, and applying Problem 8(b) to the frontier of that component. In the general case in which L and S have more than two common points, use induction on the number of these points.) (b) Prove that for any point x E ,&H), there is a simple arc of extremities 0 and x , whose points # 0 are in P(H). (“Schoenflies’s theorem.” Consider a sequence (z,) of points of B(H) such that 1z.I < p n + l , and apply (a) to two consecutive points of that sequence.)

CHAPTER X

EXISTENCE THEOREMS

There are of course many kinds of existence theorems in analysis and this chapter only deals with one kind, namely, those which are linked to the notion of completeness; roughly speaking, the most intuitive result (1 0.1.3) says that when in a Banach space the identity mapping is ‘* slightly ” perturbed by an additional term in the neighborhood of a point, it still remains a homeomorphism around that point. The word “slightly” has to be understood in a precise way, which means more than mere “ smallness ” of the perturbing function (see Section 10.2, Problem 2), and has to do with a limitation on the rate of variation of that function, generally referred to as a condition of “lipschitzian” type. As a consequence, the natural field of application of theorems of that type consists of equations in which some limitation is known on the derivatives of the given functions; furthermore, the existence theorems obtained in that way are of a local nature. In the next chapter, we will meet different kinds of existence theorems, which can be applied to global problems. The main applications of the general existence theorems of Section 10.1 are: (1) the implicit function theorem (10.2.1) together with its consequence, the rank theorem (10.3.1) which (locally) reduces to a canonical form the continuously differentiable mappings of constant rank in finite dimensional spaces; (2) the Cauchy existence theorem for ordinary differential equations (10.4.5) with its various improvements and consequences; both theorems are among the most useful tools of both classical and modern analysis. Of course, what is said of differential equations in this and the next chapter is only a tiny fraction of that vast theory; other parts of it will be examined in Chapters XXIII and XXV; the reader who wants to go farther in that direction is referred to the books of Coddington-Levinson [9], Ince [I21 and Kamke [14]. As a last application, we have given a proof of the theorem of Frobenius (10.9.4), which, as we state it, appears as a natural extension of the Cauchy 264

1 THE METHOD OF SUCCESSIVE APPROXIMATIONS

265

existence theorem to functions of several variables; it is usually formulated in a more geometric way, as an existence theorem of manifolds having at each point a given “tangent space.” We will deal with that formulation in Chapter XVIII. It goes without saying that as usual we have expressed all results for vector valued functions, so that, for instance, we practically never speak of “ systems ” of equations; it is one of the virtues of the “vector space methods” that one never need consider more than one equation, at least for the proofs of the general theorems.

1. T H E M E T H O D OF SUCCESSIVE APPROXIMATIONS

As in Chapter IX, K will denote either the real or the complex field, and whenever a statement is made about Banach spaces without specifying K, it is understood that all the Banach spaces concerned are over the same field. (10.1 .I)Let E, F be two Banach spaces, U(resp. V ) a n open ball in E (resp.F) of center 0 and radius CI (resp. p). Let v be a continuous mapping of U x V into F, such that IIv(x, yl) - u(x, y2)ll Q k * Ily, - y211for x E U, YI E V, Y Z E V , where k is a constant such that 0 Q k < 1 . Then, if IIv(x, 0)II < p(1 - k ) for any x E U, there exists a unique mapping f of’U into V such that (10.1 .I .I)

for any x

E

f ( x ) = V(X,f(X))

U; a n d f i s continuous in U.

For any x E U, we will show there exists a sequence (y,) of points of V such that yo = 0, y , = v(x, Y,-~) for any n 2 1. We have to show that if y , is defined and in V for 1 Q p Q n, then v ( x , y , ) ~ V But . we have then for 2 < p d n, y , - y p - l = v(x, Y , - ~ ) - ~ ( x ,Y , - ~ ) , hence Ily, - yp-lll < k l/yp-l- yp-2.1, and by induction on p we conclude that IY, - Y p - l l l 6 kP-lll~lll.Hence (10.1.1.2)

I(y,I( < (I

+ k + k2 +

+ kP-l)IIYIII < llyiII/(1 - k ) < P

which proves our contention. Moreover, by induction on n, we can write y , = f , ( x ) , wheref, is a continuous mapping of U into V (3.11.5). We have furthermore Ilf,(x) -f,-l(x)II Q k”-’p(l - k ) for x E U, hence the series ( f , - , f n - l ) is normally convergent (Section 7.1) in BF(U);as F is complete, the

266

X

EXISTENCE THEOREMS

series (f,(x) - f , - , ( x ) ) is convergent for any x E U, and if f ( x ) is its sum, f is continuous in U (7.2.1); moreover, by the principle of extension of inequalities applied to (10.1.1.2), I(f ( x ) / < Ilu(x,O)ll/(l - k ) < /? for any x E U, hence f is a mapping of U into V. From the relation f,(x) = u(x,f,-,(x)) we deduce (10.1.1 .I)by passage to the limit, for every x E U. Finally, suppose g is another mapping of U into V such that g(x) = u(x, g(x)) for any x E U. Then, from that relation and (10.1.1.1) we deduce IMX)

- f (4I1 = M X , g ( 4 ) - u(x, f ( X N I1 G k ' llg(x) -f (4II

and this implies g ( x ) =f ( x ) since k < 1.

(10.1.2)

(Fixed point theorem) Let F be a Banach space, V an open ball

in F of center yo and radius j?. Let u be a mapping of V into F such that Ilu(y1) - u(y2)II k * Ily, y,II for any pair of points y,, y , of V , where k is a constant such that 0 k < 1. Then, i f Ilu(yo) - yo 11 < /?(1 - k ) , there is one and only one point z E V such that z = u(z).

<

<

-

Observe that u is continuous in V and apply (10.1 . I ) to the mapping ( x ,y ) -+ u(y + y o ) - y o , which is independent of x.

(1 0.1.3) Let F be a Banach space, V an open ball in F of center 0 and radius /?. Let w be a mapping of V into F such that I[w(yl) - w(y,)(( < k . I(y, - y,(I for any pair of points of V , with k constant and 0 < k < 1. Then, i f IIw(0)II < fP(1 - k), there is an open neighborhood W c V of 0, such that the restriction to W of the mapping y + g(y) = y w(y) is a homeomorphism of W onto an open neighborhood of 0 in F.

+

We apply (10.1.1) to E = F, U being the open ball of center 0 and radius IIw(O))(, and to the mapping ( x ,y ) --+ u(x, y ) = x - w(y); the conditions of (10.1.1) are then verified, hence there is a continuous mapping f of U into V such that f ( x ) = x - w ( f ( x ) ) , in other words g ( f ( x ) )= x for x E U . To prove f is a homeomorphism of U onto f(U), we need merely to show that g is an injectiue mapping of V into g(V) (since f is clearly injective in U); but the relation g(y,) = g ( y 2 ) implies IIYI - YZII = IIw(y1) - w ( ~ ~ ) Gl l Wly, - y 2I0 hence y1 = y , since k < 1. Therefore g is the homeomorphism of W = f(U) onto U , inverse to f ; moreover W = g - ' ( U ) is open in F (3.11.4). Finally we have 0 E W, for that condition is equivalent to IIw(0)JJ< +j?(1 - k). a = p(1 - k ) -

1 THE METHOD OF SUCCESSIVE APPROXIMATIONS

267

PROBLEMS

1. Let A be a compact metric space, d the distance on A, o a mapping of A into itself such that for every pair (x, y ) of distinct points of A, d(v(x), v(y)) < d(x, y). Show that there exists a point z E A such that v(z) = z. (Use contradiction, by considering the number c = infd(x,u(x)) and proving that there exists a point Y E A such that XEA

d(y, ~ ( Y N= c.) 2. Let B be the ball JIxJJ C 1 in the space (co) of Banach (Section 5.3, Problem 5). Let u be the continuous linear mapping of (co) into itself such that

(

-

u(e,) = 1 - 2":,) e"+'

+

(n20),

+

and let u(x) = $(I Ilxll)eo u(x). Show that v is a continuous mapping of B into itself such that for any pair (x, y ) of distinct points of B, Ilu(x) - u(y)/I < IIx - yII, but that there is no point z E B such that u(z)= z (use the inequality

h(1-

tcl)

1=1

> 1-

2

I=1

cCl

for 0 < tcI

< 1).

3. Let E, F, be two normed vector spaces, u a linear homeomorphism of E, onto a subspace u(E) of F; let u : u(E) + E be the inverse mapping of u, and let m = Ilvll. (a) Let A be an open subset of E, w a mapping of A into F such that Ilw(x~)- w(Xz)II < kllxl - x211 for x I , x 2 in A. Show that if the constant k is such that km < 1 , then x -f(x) = u(x) w(x) is a homeomorphism of A onto f(A). If in addition, E and F are Banach spaces, and u(E) = F, show that f ( A ) is an open subset of F (use (10.1.3).) (b) Suppose w is a continuous linear mapping of E into F such that llwl( < ljm. Show that f is then a linear homeomorphism of E onto f(E). Furthermore, for any yo EU(E) such that llyoII = 1, show that there exists y ~ f ( E )such that lly - YOII< mllwll; conversely, for any y ef(E) such that liyll= 1 , show that there exists yo E u(E) such that Ily - ~ o I6I mllwll/(l - mllwll). 4. Let E, F be normed spaces, u a continuous linear mapping of E into F, such that N = u-'(O) is a finite dimensional subspace, then there exists a closed topological supplement M of N (Section 5.4) such that the restriction of u to M is a homeomorphism onto u ( M ) = u(E) (see 161). Let w be a continuous linear mapping of E into F;

+

+

show that if llw/l is small enough, a n d f = u w , thenf-I(O) has finite dimension at most equal to the dimension of u-'(O), and there is a closed topological supplement P of f - ' ( O ) , such that the restriction off to P is a homeomorphism onto f(P) =f(E) (use Problem 3(b)). 5. Let E be a Banach space, F a normed space, u a linear homeomorphism of E onto u(E) such that there is a topological supplement Q of u(E) in F. Show that if w is a continuous linear mapping of E into F with sufficiently small norm llwll, and f = u w, then Q is still a topological supplement of f(E) in F. (Show that the projection of F onto u(E), restricted tof(E), is a linear homeomorphism onto u(E) when llwll is small enough, using Problem 3.) 6. Let E, F be two Banach spaces, i4 a continuous linear mapping of E into F such that N = u-'(O) has finite dimension p ; then N has a topological supplement M such that the restriction of u to M is a homeomorphism into u(M) = u(E). Suppose in addition u(E) has finite codimension q in F. Let w a continuous linear mapping of E into F;

+

268

X

EXISTENCE THEOREMS

+

7.

show that if llwlj is small enough, f = u w is such that the dimension r of f-’(O) satisfies the inequalities p - q < r < p , and that the codimension of f(E) in F is equal to q - p r. (Use Problems 4 and 5, as well as (5.9.3).) Let I = [0,1], and let P be the subspace of the Banach space CR(I) (Section 7.2) consisting of the restriction to I of the polynomials x(t) with real coefficients. In the normed space P, let u be the identity mapping x x, and let w be the linear mapping which to each polynomial x(r) (restricted to I) associates the polynomial x(t2), restricted to I. For any E such that 0 < E < I , the linear mapping f = u EW is a linear homeomorphism of P onto the subspacef(P), but the codimension of f(P) in P is infinite (compare to Problem 6). Let E, F be two Banach spaces, u a continuous linear mapping of E into F such that u(E) = F ; then there exists a number m > 0 such that for any y E F there is an x E E for which u(x) = y and llxll < mllyil (12.16.10). Let w be a continuous mapping of an open ball U = B(a; r) c E into F, such that Ilw(xd - w(x2)ll < kllxl - x 2 II for XI, x2 in U. Prove that if k and Ilw(a)ll are small enough, the continuous mapping x + f ( x ) = u(x) w(x) is such that f(U) contains an open ball of center u(a). (Use the same method as in the proof of (lO.l.l).) Suppose E, F, U, V, and u satisfy the assumptions of (10.1.1). In addition, let q~ be a continuous mapping of U into itself such that Ilp(x) I1 < ((XI/ for any x E U. Show that (under the condition Ilu(x, O)ll< p(1 - k ) for x E U) there exists a unique mapping f of U into V such thatf(x) = u(x,f(v(x))) and that f i s continuous in U. Generalize to equations of the form f(x) = u(x,f(vI(x)), . ,f(vP(x))). Let E, F, U, V have the same meaning as in (10.1.1). Suppose the continuous mapping u of U x V into F satisfies the following conditions:

+

--f

+

8.

+

9.

..

10.

< c(llx/12”+ IlylllZ”+ llY21I2’)llY1

(1)

IIu(x,yd- V(X,Y2)ll

(2)

Ilu(x, 0)Il G c/l~111+2”,

-y2Il;

where c and p are constants >O. Let h be an element of K such that 1h1 > I . Finally, ) a conlet x +L(x) be a continuous linear mapping of E into F, and let x + ~ ( xbe tinuous mapping of U into itself such that lIp(x)ll < llxll. Show that there exists a mapping f of a neighborhood W c U of 0 into V, having the following properties: (1)fsatisfies in W the equation

(2) lim ( f ( x )- L(x))/Ilxll= 0. Furthermore, any two mappings having these prop*-0, X # O

erties coincide in a neighborhood of 0. (Reduce the problem to the case in which L(x) = 0. Observe that if f satisfies the preceding conditions, then one must have in a neighborhood of 0

where the series is normally convergent in a neighborhood of 0. Then use the method of (10.1.1) to prove the existence of a solution of (*) in a sufficiently small neighborhood of 0; show, by induction on n, that there exists an r > 0 such that, for JJxll < r, llf.(x)II < IIxlll+”and llfn(x)-fn-~(x)II < Ilxll’+””).

1 THE METHOD

OF SUCCESSIVE APPROXIMATIONS

269

11. (a) Let F(xl, ..., x p , y ) be an entire function in Kp+', such that in the power series equal to F ( x l , ... , x r , y), all monomials have a total degree 2 2 . Let p be a linear mapping of Kp into itself such that Ilp(x)il < J/xl/for any x = (xl,. .. ,xp)E Kp; finally, let L(x) be an arbitrary linear form on Kp. Show that there is a unique solution f of the equation

f(x) - hfif(x/X)= F(x,f(p(x)))

(Ihl > 1)

which is defined in a neighborhood of 0 and such that lim(f(x)-L(x))/llxll = O . X-0

Furthermore, that solution is an entire function in Kp. (Apply Problem 10 in a neighborhood of 0; reduce the problem to the case K = C, and apply (9.4.2) and (9.12.1) to prove that f i s an entire function.) (b) Show that there is no solution of the equationf(x) - hf(x/h)= x (h > 1 ) defined in a neighborhood of 0 in R and such thatf(x)/x is bounded in a neighborhood of 0. 12. Let I = [O, a ] , H = [ - b , b ] , and let f be a real valued continuous function in I x H; put M = sup If(x, y)l, and let J = [0, inf(a, b / M ) ] . (X.Y) E

1X H

(a) For any x E J, let E(x) be the set of values of y E H such that y = xf(x, y). Show that E(x) is a nonempty closed set; if g l ( x ) = inf(E(x)), g2(x) = sup(E(x)), show that gl(0) = gz(0)= 0, and that lim gl(x)/x= lim g2(x)/x =f(O, 0). x-o.x>o

X+O.X>O

If g1= gz = g in J, g is continuous (cf. Section 3.20,Problem 5). (b) Suppose a = b = 1 ; let E be the union of the family of the segments S. : x = 1/2", 1/4"+'< y 6 1/4" (n 2 0), of the segments S. : y = 1/4, 1/2"< x C 1/2"-' (n 3 ])and of the point (0,O). Define f ( x , y ) as follows: f ( 0 , y ) = 0; for 1/2"< x C 1/2"-' and y S 1/4", takef(x, y ) = ( ( y / x ) d((x, y), E))+ ;for 1/2"< x C 1/2"-' and 1/4" < y < xz, take f ( x , y ) = ( y / x ) - d((x, y), E) and finally, for 1/2"< x < 1/2"-' and y C x 2 , takef(x, y ) = x - d((x, xz), E) (n 3 I). Show thatfis continuous, but that there is no function g, continuous in a neighborhood of 0 in I and such that g ( x ) = xf(x, g(x)) in that neighborhood. (c) Let u0 be a continuous mapping of J into H, and define by induction u.(x) = x f ( x , U " - ~ ( X ) ) for n 3 1 ; the functions u, are continuous mappings of J into H. With the notations of (a), suppose that in an interval [0, c ] C J, lim ( U , , + ~ ( X )- u.(x)) =

+

"-m

0 for every x ,

and g l ( x ) =gz(x); show that lim u,(x) = g l ( x ) for 0 < x C c. n-ro

Apply that criterion to the two following cases: (1) there exists k > O such that If(x, zl) - f ( x , zz)[ C k Izl - z21 for x E I, zl, zz in H (compare to (10.1.1)); (2) for 0 < x < y < a and zl, zz in H, ]f(x,ZI)-f(x, Z Z ) ~< I Z I - JZ(/X. (d) Whenfis defined as in (b), the sequence (u,(x)) is convergent for every x E I, to a limit which is not continuous. (e) Take a = b = 1, f ( x , y ) = y / x for O < x C 1,lyl < x 2 , f ( x , y ) = x for 0 < x < 1,y 2 x',f(x,y) = - x for 0 C x < I , y < -xz. Any continuous function g in I such that [g(x)l < xz is a solution of g ( x ) = xf(x, g(x)) although If(x, zl) - f ( x , zz)/ < Izl - zzI/x for 0 < x < 1, zI, zZ in H ; for any choice of u O ,the sequence (u.) converges uniformly to such a solution. (f) Definefas in (e), and let f l ( x , y ) = - f ( x , y). The function 0 is the only solution of g(x) = x f l ( x ,g(x)), but there are continuous functions uo for which the sequence (u,(x)) is not convergent for any x # 0, although Ifl(x, zl) - f i ( x , z2)I < [zI- zzI/x for O < x < l,zl,zzinH. 13. Generalize the results of Problems 12(a) and 12(c) when H is replaced by a disk of center (0,O) in R2 (use the result of Problem 3 of Section 10.2).

270

X

EXISTENCE THEOREMS

2. IMPLICIT FUNCTIONS

(The implicit function theorem) Let E, F, G be three Banach (10.2.1) spaces, f a continuously di'erentiable mapping (Section 8.9) of an open subset A of E x F into G. Let (xo ,yo)be a point qf A such that f (x,,yo) = 0 and that the partial derivative D , f (xo,yo) be a linear homeomorphism of F onto G. Then, there is an open neighborhood Uo of xo in E such that, for every open connected neighborhood U of xo , contained in Uo , there is a unique continuous mapping u of U into F such that u(xo) = y o , (x,u(x)) E A and f (x,u(x)) = 0 for any X E U. Furthermore, u is continuously di'erentiable in U, and its derivative is given by (10.2.1 .I ) u'(x) = - ( D , f (x, 4 x 1 ) ) - W l f (x,W ) ) . Let To be the linear homeomorphism D, f ( x o ,yo) of F onto G, T i 1 the inverse linear homeomorphism; write the relation f ( x , y ) = 0 under the equivalent form (1 0.2.1.2) y =y - Ti1 . f ( x .y ) and write g(x, y ) the right-hand side of (10.2.1.2). We are going to prove that it is possible to apply (10.1.1) to the mapping

(x'9 Y'>

+

S(X0

+ x', Yo + Y ' ) - Y o

of E x F into F, in a sufficiently small neighborhood of (0,O).As T;'. To = 1 by definition, we can write, for ( x , y,) and ( x , y,) in A,

-

(Dzf(x0 Y o ) ( Y , - Y 2 ) - (f(x, Y1) -f(& Y,))). Let E' > 0 be such that E(IZ';'(~ G 3; as f is continuously differentiable in A, it follows from (8.6.2) and (8.9.1) that there is a ball Uo (resp. V,) of center x , (resp. y o ) and radius c1 (resp. p) in E (resp. F) such that, for x E U, ,y1 E V, ,y 2 E Vo , we have g(x9 Y , )

- g(x,

Y2)

I l f ( x ,Yl)

=Ti'

*

3

-m,YZ) - D2f(xo Y o )

- Y2)ll EllYl - Y2II ; whence Ildx, Y1) - g(x, Y2)Il G E I I T i ' II * llYl - Yzll G -511Yl - Y2lI for any x E U, ,y , E Vo ,Y,EV,. On the other hand, g(x, yo) - yo = - Ti1- f ( x , yo); 3

*

(Y1

as f ( x o , yo) = 0 and f is continuous, we can suppose E has been taken small enough to have 11g(x, yo) - yoI) G p/2 for x E Uo . We can then apply (10.1.1 ), which yields the existence and uniqueness of a mapping u of Uo into V, , such that f ( x , u(x)) = 0 for every x E U,; asf(xo ,yo) = 0, this gives in particular u(xo)= y o ; finally u is continuous in U, . Next we prove that if U c Uo is a connected open neighborhood of xo , u is the unique continuous mapping of U into F such that u(xo) = y o , ( x , u(x))E A and f ( x , u(x)) = 0. Let v be a second mapping verifying these

2 IMPLICIT FUNCTIONS

271

conditions, and consider the subset M c U of the points x such that u(x) = u(x). This set contains x, by definition and is closed (3.15.1); we need therefore only prove M is open (Section 3.19). But by assumption, x -+ DZf(x, u(x)) is continuous in U, , hence (replacing if necessary U, by a smaller neighborhood), we can suppose that D2f(x, u(x)) is a linear homeomorphism of F onto G for x E U,, by (8.3.2). Let a E M ; the first part of the proof shows that there exists an open neighborhood U, c U of a and an open neighborhood V, c V of b = u(a) such that, for any x E U,, u(x) is the only solution y of the equation f(x, y ) = 0 such that y E V,, . However, as u is continuous at the point a, and u(a) = u(a), there is a neighborhood W of a, contained in U , and such that ~ ( x E) V, for x E W ; the preceding remark then shows that u(x) = u(x) for x E W, and this proves M is open, hence u = u in U. Finally we show that u is continuously differentiable in U, , provided E has been taken small enough. For x and x + s in U,, let us write t = u(x + s) - u(x); by assumption f(x s, u(x) + t ) = 0, and t tends to 0 when s tends to 0. Hence, for a given X E U , , and for any 6 > 0, there is r > 0 such that the relation llsll < r implies Ilf(x s, u(x) t ) -f(x, u(x)) - S(x) . s - T(x) tll < S(llsll Iltll) where S(x) = D,f(x, u(x)) and T ( x ) = D2f(x, u(x)) (8.9.1). This is equivalent by definition to

+

+

+

+

IIS(x) * s + T(X) * 41 < 6(llsll

+ Iltll)

and as T(x) is a linear homeomorphism of F onto G , we deduce from the preceding relation (10.2.1.3)

ll(T-'(x) 0 S(x)) * s + tll < ~ l l ~ ~ 1 ~ x ~+lIltll). l~~141

Suppose 6 has been taken such that 6llT-'(x)ll <+; then, if we put a = 211T-'(x) 0 S(x)II 1, we deduce from (10.2.1.3) that

+

< a)lsll,and therefore Ilt + v-'w X X ) ) SII < + l)ll~-l(xN. Ibll as soon as llsll < r. By definition of t , this proves u is differentiable at the i.e.,

Ilt)l

O

*

point x and has a derivative given by (10.2.1.1). Using (8.3.2) and (8.3.1), formula (10.2.1.I) then proves u is continuously differentiable in U, . We formulate explicitly the most important case of (10.2.1), i.e. the one in which E = K"', F = G = K" are finite dimensional spaces:

272

X

EXISTENCE THEOREMS

(10.2.2) Let f i be n scalar functions defined and continuously diflerentiable in

a neighborhood U x V of a point (a,, . . . , a,, b,, . . . ,b,) of E x F, such that W l , . . .,fJ J;.(a,, . . . ,a,, b,, . . . , b,) = 0 for 1 < i < n, and that the jacobian

d ( Y 1 , . * . Y.) is not 0 at (al, . . . , a,, b,, . . . , b"). Then there is an open neighborhood Wo c U of ( a l , . . . , a,) such that, for any connected open neighborhood W c W o of (a,, . . . , a,), there is a unique system of n scalar functionsg, (1 < i < ti), de$Hed and continuous in W and such that gi(a,, . . . , a,) = bi for 1 < i < n, and

. . . , x,, gl(x1, . . . , X,), . . . , gn(xl,. . ., x,)) = 0 for 1 < i < n and any (x,, . . . , x,) E W. Moreover, the functions g i are confi(X1,

tinuously differentiable in W, and the jacobian matrix (Djg,(x)) is equal to - B - ' A , where A (resp. B ) is obtained by replacing y i by gi(x,, . . . , x), (1 < i < n) in the jacobian matrix (dfi/dx,) (resp. (df;/dyj)). (10.2.3) If the assumptions of (10.2.1) are verified, and if in addition f is p times continuously diflerentiable in a neighborhood of (xo,yo), then u is p times continuously diflerentiable in a neighborhood of xo .

We prove by induction on k that u is k times continuously differentiable for I < k < p ; for k = 1, this follows from (10.2.1), and moreover u'(x) = F(x, u(x)), where F(x, y ) = - (D, f ( x , y))-' 0 ( D , f ( x , y ) ) is p - 1 times continuously differentiable by (8.12.9), (8.12.11), and (8.12.10). By (8.12.10), u' is therefore k - 1 times continuously differentiable (for k < p ) and that means that u is k times continuously differentiable by (8.12.5). (1 0.2.4) Suppose E, F , G are finite dimensional, and f is analytic in A ; then u is analytic in a neighborhood of

xo .

If the field of scalars K is C, the result follows from (10.2.1) and the characterization of analytic functions as continuously differentiable functions (9.1 0.1). Suppose now K = R,E = R", F = G = R"; then there is an open set B c C"'" such that B n R"'" = A and an analytic mapping g of B into C" which extends f (9.4.5). Identifying D, f and D , g with jacobian matrices shows that D,g(xO,yo) transforms a basis of R"over R into a basis of R", and these bases are also bases of C" over C, hence D, g(xo,yo) is a linear homeomorphism of C" onto itself. We therefore can apply (10.2.1) tog, which shows the existence of an analytic mapping u of a neighborhood W of xo in C" such that g(z, u(z)) = 0 and u(xo) = y o . Moreover, it follows from formula (10.2.1 . I ) by induction on ) I ! ) that all the derivatives D'ZIat the point xo map R" into

2 IMPLICIT FUNCTIONS

273

R" (since all derivatives of g at ( x , ,yo) are equal to the corresponding derivatives off); hence, by (9.3.5.1), v maps a neighborhood of x , in R" into the space R", and the uniqueness part of (10.2.1) therefore proves that the restriction of v to W n R" is identical to u. Q.E.D.

One of the most important applications of (10.2.1) is the following:

(10.2.5) Let E, F be two Banach spaces, f a continuously diferentiable mapping of a neighborhood V of x, E E into F. If f'(x,) is a linear homeomorphism of E onto F, there exists an open neighborhood U c V of x, such that the restriction off to U is a homeomorphism of U onto an open neighborhood of y o =f (x,) in F. Furthermore, iff is p times continuously diferentiable in U (resp. analytic in U, E and F beingfinite dimensional), the inverse mapping g off (U) onto U is p times continuously diferentiable (resp. analytic) in f (U).

Apply (10.2.2) to the function h(x, y ) = f ( x ) - y , exchanging the roles of

x and y ; as D,h(x, ,yo) =f '(x,), we conclude that there is an open ball W

of center y o in F and a continuous mapping g of W into E such that g ( W ) c U, f ( g ( y ) )= y in W and g(y,) = x,; furthermore, by (10.2.3) (resp. (10.2.4)), if f i s p times continuously differentiable (resp. analytic), gisp timescontinuously differentiable (resp. analytic). From the identity f (g(y))= y it follows that g is injective in W, hence is a bijective continuous mapping of W onto V = g ( W ) c U; moreover, g ( W ) = f - ' ( W ) is open in E, and f is a homeomorphism of V = g ( W ) onto W , which ends the proof.

PROBLEMS 1. Let E, F be two Banach spaces, A an open neighborhood of a point x, E E, f a continuous mapping of A into F, which is differentiable at xo (but not necessarily at other points of A). Suppose f'(xo) is a linear homeomorphism of E onto its image in F; show that there is a neighborhood U c A of xo such that f(x) #f(xo) for every x E U such that x # xo . (Observe that the assumption implies the existence of a constant c > 0 such that Ilf'(xo) . s l l 3 cI/sI/ for all s E E (5.5.1)J 2. Letf= (ft,f2) be the mapping of RZ into itself defined byf,(xl, x2) = xl;f2(x1,x2) = x2 - x: for x: < x 2 , f2(xl, x 2 ) = (x: - x:x2)/x: for 0 < x2 < x:, and finally fz(x1. - XI) = -fz(xt, x2) for x2 0. Show that f is differentiable at every point of RZ;at the point (0, 0), Df is the identity mapping of RZ onto itself, but Df is not continuous. Show that in every neighborhood of (O,O), there are pairsof distinct points x', x" such that f ( x ' ) = f ( x " ) (compare to (10.2.5)).

274

X

EXISTENCE THEOREMS

+

3. Let B be the unit disc IzI f 1 in R2 and let z -f(z) = z g(z) be a continuous mapping of B into R2 such that lg(z)l < Izl for every z such that JzI= 1. Show that f(B) is a neighborhood of 0 in R2 (“Brouwer’s theorem” for the plane, cf. Chapter XXIV). (Let y be the loop t - f ( e “ ) defined in [0,27r]; show thatj(x; y ) = 1 for all points x in a neighborhood V of 0 (see proof of (9.8.3)); using the fact that, in B, y is homotopic to 0, deduce that there is no point of V belonging to the complement off(B).) 4. Let E, F be two Banach spaces, B the unit open ball /lx/l< 1 in E; let uo be a continuously differentiable homeomorphism of B onto a neighborhood of 0 in F, such that uo(0)= 0; suppose uo’ is continuously differentiable in a ball Vo : llyll< Y contained in uo(B), and Duo is bounded in B and Duo’ is bounded in V,. Let V be a ball llyll < p, with p < r. (a) Show that for any a < 1, there is a neighborhood H of uo in the space 9 P ) ( B ) (Section 8.12, Problem 8) such that for any u E H, the restriction of u to U: llxll< a is a homeomorphism of U onto an open set of F containing V, such that the restriction of u-’ to V is a continuously differentiable mapping @(u) of V into E. (Use (lO.l.l).) (b) Show that the mapping u @(u) of H into 9(EI)(V)is differentiable at the point uo ,and that its derivative at uo is the linear mappings + -(uk 0 cD(uo))-’ . (s 0 cD(uo)). 5. Let E, F be two Banach spaces, f a continuously differentiable mapping of a neighborhood V of xo E E into F. Suppose there are two numbers /?> 0, A > 0 such that: (I) Ilf(x0)II < /3/2h;(2) in the ball U: IIx - xoIJ< p, the oscillation off’ is < l/2A; (3) for every x E U,f’(x) is a linear homeomorphism of E onto F such that ll(f’(x))-’ I1 < A. Let (2,) be an arbitrary sequence of points of U ; show that there exists a sequence (x.).,~ of points of U such that x.+’ = x. - (J’(zn))-’ .f(x,) for n 2 0. Prove that the sequence (x,) converges to a point y E U, such that y is the only solution of the equationf(x) = 0 in U. (“Newton’s method of approximation.” Use (8.6.2) to prove by induction on n that IIx, - x.-’ I/ < 2-”p and Ilf(xn)II < /3/2“+‘X). 6. Let E, F be two finite dimensionalvector spaces over K, A a connected open subset of E, f a continuously differentiable mapping of A x F into F. Suppose that the set r of pairs (x, y ) E A x F such f(x, y ) = 0 is not empty, and that for any (x, y) F I?, D2f(x, y ) is an invertible linear mapping of F onto itself. (a) Show that for every point (xo ,yo ) E r there is an open neighborhood V of that point in r such that the restriction of the projection prl toVis a homeomorphism of V onto an open ball of center xo contained in A. (Use the fact that there is an open ball U of center xo in A and an open ball W of center yo in F such that for each x E U, the equationf(x, y) = 0 has a unique solution y E W, and apply (10.2.1).) (b) Deduce from (a) that every connected component G of r (Section 3.19) is open in I’ and that pr,(G) is open in A. It is not necessarily true that pr,(I’) = A (as the example A = E = F = R,f(x,y) = xy2 - 1 shows), nor that if prl(r) = A, prl(G) = A for every connected component G of r (as the example A = E = F = R,f ( x , y ) = xy2 - y shows). Prove that if pr2(I’) is bounded in F, then pr,(G) = A for every connected component G of F. (If xo is a cluster point of pr,(G) in A, show that there is a sequence (x,, y.) of points of G such that lim x, = xo and that limy. exists in F; n-1 m n-m apply then (a).) (c) The notations of path. loop, homotopy, and loop homotopy in A are defined as in Section 9.6, replacing C by E. Suppose there is a connected component G of r such that prl(G) = A; if y is a path in A, defined in I = [a, b] c R,show that thereexists a continuous mapping u of I into G such that prl(u(r)) = y ( t ) for each t E I (consider the 1.u.b. c in I of the points 6 such that there exists a continuous mapping uc of [a, 61 into G such that prl(uc(r)) = y(f) for a < t < 5 and use (a)). Is that mapping always unique? (Consider the case E = F = C, A = C - { O } , f ( x ,y ) = y 2 - x . ) --f

2 IMPLICIT FUNCTIONS

275

Show that if two continuous mappings u, u of I into G are such that prl(u(t)) = prl(u(t)) = y(r) for each t E 1,and if they are equal for one value of t E I, then u = u (use a similar method). (d) Under the same assumptions as in (c), let v be a continuous mapping of I x J into A, where J = [c, d ] C R. Let u be a continuous mapping of J into G such that prl(u([)) = ~ ( a5), for 6 E J ; and for each 5 E J, let uc be the unique continuous mapping of I into G such that prl(uc(t)) = ~ ( t , tfor ) t E I and u&) = ~(6).Show that the mapping ( t , 5) + u,(t) is continuous in I x J. (Given 5 E J, there is a number r > 0 such that for any t E I, the intersection V, of I' and of the closed ball in E x F, of center u,(t) and radius r, is contained in G and such that prl is a homeomorphism of V, onto the closed ball in E of center y ( t ) and radius r. If L = u{(I), let M be the supremum of ll(D2f ( x , y))-' 0 ( D , f ( x , y))/I for all points ( x , y) E G at a distance < r of L. Let E > 0 be such that E < 1-14and E M < r/4. Show that if 6 is such that the relation 15 - 51 < 6 implies Ilq(t,5) - q ( t , <)I] < E for t E I, then the relation 16 - (1 < 6 implies llu,(t) - ui(t)Ij < r/4 for t E I ; prove this by considering the 1.u.b. of the 1 E I for which the inequality holds, and using (10.2.1).) (e) Conclude from (d) that if the loop y defined in I = [n, b] is loop homotopic to a point in A, then any continuous mapping u of I into G such that prl (u(t))= y(r) for t E I is such that u(b) = u(a). In particular, if A is simply connected (i.e,, if any loop in A is homotopic to a point in A), then prl is a homeomorphism of G onto A, i.e. there exists a unique continuously differentiable mapping g of A into F such that f(x, g(x)) = 0 in A and that ( x , g(x)) belongs to G for at least one x E A (cf. (16.28.7).) 7. With the notations of Problem 6, show that the condition prl(C) = A is satisfied for every connected component G of I' in each of the following cases: (1) f(x, y) = f i ( y ) - f z ( x , y), and there exist numbers R > 0, k > 0, h > 0 and a positive continuous function x + H(x) in A such that for llyll > R, Ilfi(y)II 2 lIyIIk and Ilfz(x,v)ll < H ( x ) l l ~ / l ~ - ~ . ( 2 ) F = C, E is a vector space over C , f ( x , y) = e y - g(x), where g is analytic in A andg(x) # 0 in A (this last condition already ensures that pr,(I') = A; observe that f ( x , y) = f ( x , y') implies that y' - y is a multiple of 2ni, hence for any x E A there is an open ball U of center x contained in A such that for any connected component V of pr;'(U) n I',prl is a homeomorphism of V onto U; if x is a cluster point of prl(G), G must have a common point with one of these components V, hence contains V). 8. (a) Iffis a complex valued entire function in C p ,such that f ( x ) # 0 for every x E C p , show that there exists a complex valued entire function g in C p such that f ( x ) = e g c x ) (use Problem 7). (b) Letf be an arbitrary complex valued entire function in C, which is notidentically 0; there is a finite or infinite sequence (a,) (with n > 1) of complex numbers (which may be empty) such that /n,l < lanfl/,f(n.) = 0 and for every c E C such thatf(c) = 0, the number of indices n for which a, = c is equal to the order w ( c ; f ) ;when the sequence (an) is infinite, lim lanl = +co (9.1.5). Show (with the notations of Section 9.12, n-m

Problem 1 ) that there exists an entire function g such that

fi E (?, n a,

f ( z ) = egc2)

n=1

-

I),

(" Weierstrass decomposition ").

9. Let A and B be two open neighborhoods of 0 in E = C p ,A being connected; let ( x , y) + U(x,y) be an analytic mapping of A x B into Y(E; E) (identified to the space of p x p matrices with complex elements).

276

X

EXISTENCE THEOREMS

(a) Suppose there exists a sequence (u,,) of analytic mappings of A irito B such that rro(x) -= 0 in A and r/,,(.u) U ( s , r/,8-i(.r)). x in A for Ir > I . Suppose in addition that for every compact subset L of A, the restrictions of the I(,, t o L form a relatively compact subset of 'LE(L). Prove that the sequence ( I ! , ) converges uniformly in any compact subset of A to an analytic mapping u of A into B such that u ( x ) U(.u, r(.u)) ' x in A ; furthermore, u is the unique mapping satisfying that equation (use (10.2.1) and ~~

~~~

(9.13.2)).

(b) Suppose that in E, A and B are the open balls of center 0 and radii u and 7)) Let rp be a continuous mapping of [0, ( I [ Y [0, h[ into R such that 7 increasing in [O, / I [ for every [ E [O, ul and suppose that IIU(x, y)Il < ~(ll.rll,IIJII) in A x' B. Suppose in addition that there exists a continuous mapping 0 of [0, N [ into [0, h[ such that =,[(pr &[))[ in [O, u ] . Prove that under these conditions there is a unique analytic mapping u of A into B such that ds) U ( x , dx)) ' sin A, and in A (use (a); prove the existence of the mappings I/,, by induction that Ilo(x)li3 ~(!lsll) on ti). (c) Suppose A and B are defined as in (b): let #(7) be the 1.u.b. of llU(x,.v)Il for l!xIi < a , ilyll -< 7, when 7 : 0, and take 440) $(O i ). Suppose that #(O) 0 and that the function 7 -t 7 / $ ( ~is )increasing in some interval [0, y [ , where y < / I , and Y/I,!J(Y-) < a. Then there is a unique analytic mapping I ) of the open ball P of center 0 and radius y/i,b(y-) into B, such that u(x) U ( x , dx)) . s in 1'. 10. Let f , g be two complex valued analytic functions defined in a neighborhood of the closed polydisk P c Cz of center (0,O) and radii u , h. Let M (resp. N ) be thc 1.u.b. of y)i (resp. lg(x, y ) l ) for Is1 - N and IyI < /I (resp. for 1x1 < N and Iyi --= 1)). Then, there exist two uniquely determined functions u(,s,t ) , o(s, t ) , analytic for 1 . ~ 1 '- tr/M and jtl .::DIN, such that ( ~ ( s , t ) , t f s , t ) )c P for (.Y, t ) in the polydisk Q defined by tlic previous inequalities and that

-v([,

s(()

7

I

: :

~

~ ( s ,t ) - .vf(u(.s,

t ) , c(s,

1))

0

7

and

u ( s , t ) - tg(u(.s, I ) ,

P(S,

I)) -0

in Q. Furthermore, let

and let h(x,y, s, t ) be a n arbitrary analytic function in P

~,

0 ; show that

h(rr(s, t ) ,u(s, I ) , s , I ) = C c,,,,.s"'/" A(u(.s, f ) , P ( S , t ) , . ~t, ) r r t 3 0 . n ~ 0

for

(s,

t ) E Q, where crnn is the value for .r

~

J'

=I

0 of the function

and the series o n the right-hand side is convergent in Q ; note that c,,,. depends o n .s and t if h does. (" Lagrange's inversion formula." First apply RouchC's theorem (9.17.3) to s - sf(x, y), considered as a function of x; this defines an analytic function W(S, y) such that w(s, y ) - sf(w(s,.~),y ) = 0, by (10.2.4); next apply similarly Rouche's theorem to y tg(w(s, y ) . .v) considered as a function of y . Finally, let y , 6 be the . the repeated integral circuits 0 ->acio, 8+hei0 in C (0 < 0 < 2 ~ ) Consider -

3 THE RANK THEOREM

.r,

i,

277

h ( x , y , S , t ) dx (x

- .Sf(S,

y))(y

~

/g(x, y))

.

On one hand, find the value of that integral by repeated application of the theorem of residues (9.16.1); and on the other hand, consider the power series development of (I [)-'(I 7)Vtin which [ is replaced by sf(x, . v ) / x and 7 by / g ( x , y)/y.) Generalize to any number of complex variables. From the inversion formula for one variable, deduce the formula ~

~

h(u(.s)) where

I/(.Y)

~

cf(r/(s))

=

~

h(0)

0 and

Is/

+f "=I

/I!

Dn-l(h'(0)(f(O))")

< n / M , with M

=

sup If(x)l, h being analytic for 1.~16a

1.Vl ~, (1.

3. T H E RANK T H E O R E M

Let E, F be twoJinite tliniensional vector spaces of dimensions n and m , A an open subset of E, f a continuously differentiable mapping of A into F, The rank of the linear mappingj"(x) at a point x E A is the largest number p such that there is at least a minor of order p in the matrix off'(x) with respect to two bases of E and F, which is not 0 (A.7.3). As these minors are continuous functions of x, it follows that if the rank of,f'(x,) is p , there is a neighborhood of xo i n which the rank o f f ' ( x )is at least p ; but it can be > p at every point x # xo of that neighborhood, as the example of the mapping (-Y, J,) (x2 - y 2 , xy) shows at the point (0,O). --f

(10.3.1) (Rank theorem) Let E be an n-dimensional space, F an m-dimensional space, A an open tieigliborlioocl of a point a E E, , f a continuously dtfferentiable mapping (resp. q times contiiiuously differeiitiable mapping, resp. indefinitely differentiable mapping, resp. analj!tic mapping) of A into F, such that in A the rank of f ' ( x ) is a constant number p , Then there exists: ( 1) an open neighborhood U c A ofa, and a homeomorphism u of U onto the unit ball I" : Ix,I < 1 ( 1 i t i ) in K", ttAic1i is continuously differentiable (resp. q times cotitinuously differentiable, resp. indefinitely differentiable, resp. analytic) as well as its inivrse ; (2) an open neighborhooci \I I> j ( U ) of b = f ( a ) and a homeomorphism u of the unit ball 1" : 1y,1 < I ( 1 i m> of K" onto V, which is continuously cliffi~rentiahle( resp. q times continuously differentiable, resp. indefinitely dtfferentiable, resp. analj~tic)as 1t.ell as its itirersc I. -such that f ' = I' f b u, n h w f o is the mapping

< <

< <

0

(XI,

of 1" into 1".

0

. . . , x,)

+ (x,,

. . x p , 0, . . 7

* * 9

0)

278

X

EXISTENCE THEOREMS

We write the proof for continuously differentiable mappings, the modifications in the other cases being obvious. We may supposea = 0,b = 0,replacingfbythemappingx - + f ( a + x) - h. Let M be the kernel of the linear mappingf'(O), which is an ( n - p)-dimensional subspace of E, and let N be a(p-dimensional) supplement of M in E; we take as a basis of E a system (cJli ii,, of 17 vectors such that cl, . . . , c p form a basis of N, c,,+~,. . . , c, a basis of M , and we write x = x E E, the

'pi

n

i= 1

q i ( x ) c ifor any

being linear forms. If el, . . . , en is the canonical basis of K", we

denote by x -+ C(x) the linear mapping x -+

2

q i ( x ) e i of E onto the

i=p+ 1

subspace KnPPof K" generated by the e i of index i > p . Let P be the image of E (and of N ) by the linear mappingf'(0); it is a p-dimensional subspace of F, having the elements di = f ' ( O ) * ci (1 Q i Q p ) as a basis; we take a basis ( d j ) l s j i , of F, of which the preceding basis of P form the first p elements, and we write y

m

=

$,(y)dj for any y

E

F, the

j=l

tjj being linear forms. We denote by y - + H ( y ) the linear mapping

y+

P

C tjj(y)ejof F onto the subspace KP of K" generated by the ei of index

j=l

i
+

We now consider the mapping x -+g(x) = H ( f ( x ) ) G(x) of A into K" which is continuously differentiable. Moreover, by (8.1.3) and (8.2.1 ), we have g'(x) s = H ( f ' ( x ) . s) + G(s) for any s E E, hence g'(0) . ci = e, for 1 < i Q n (i.e. g'(0) is represented by the unit matrix with respect to the bases ( c i )and ( e i ) ) .Using (10.2.5), we conclude that there is an open neighborhood U, c A of 0 such that the restriction of g to U, is a homeomorphism of U, onto an open neighborhood of 0 in K", and that the inverse homeomorphism g-I is continuously differentiable in g(U,). Let r > 0 be such that the ball lxil < r (1 < i < n) is contained in g(U,), and let U be the inverse image of that ball by g, which is an open neighborhood of 0 ; our mapping u will be 1

1

the restriction to U of the mapping x -+ - g(x). r

Up to now we have not used the assumption that the rank off'(x) is constant in A ; this implies that the image P, of E byf'(x) has dimension p for any x E A. Now we may suppose U, has been taken small enough so that g'(x) is a linear bijection of E onto K" for x E U, (8.3.2); as we have g'(x) * s = H ( f ' ( x ) . s) for s E N, the restriction of f ' ( x ) to N must be a bijection of that p-dimensional space onto P,, and the restriction of H to P, a bijection of P, onto KP. Denote by L, the bijection of KPonto P,, inverse of the preceding mapping; we can thus writef'(x) = L, H o f ' ( x ) . 0

3 THE RANK THEOREM

279

Now consider K" as the product El x E, with El = KP, E, = K"-P. we are going to prove that the mapping (zl, z 2 )+fl(zl, 2 , ) = f ( u - ' ( z l , z2)) of 1" into F does iiof depend 017 z2 , i.e. that D, fl(zl, z , ) = 0 in I" (8.6.1). By defini-

rf'(x> . t

'

i:

= f, - H(J'(x)),-

tion, we can writef(x)

r

= Dlfl(;

+ D2 f i for any t E

(10.3.11)

E. This yields D, f I

G(,4), hence by (8.9.2)

; ;w).

H ( f ( x ) ) , G(x)) . W f ' ( x ) . t )

(;

H(S(x)),

G(0

;

(i

H ( f ( x ) ) , G(x)) . G ( t ) = S, * H ( f ' ( x ) . t )

' 1

(i

, G(x) is a linear mapping of KP = El where S, = rL, - Dlfi - H ( f ( x ) ) r

into F. We prove that S, = 0 for any x E U,. Indeed, if t E N, we have G ( t )= 0 by definition, hence S, . H ( f ' ( x )* t) = 0 by (10.3.1.1). But t -+ H ( f ' ( x ) . t ) = g'(x) * t is a bijection of N onto El for x E U,, and this proves S, = 0. From (10.3.1.1) we then deduce

D, for any

f E

fl

(' r

I

H(J'(x)),- C(x)) r

. G(t) = 0

E; but G maps E onto E,, hence by definition,

which is a linear mapping of E, into F, is 0 for any x E U,, . The relation D,f,(z,, z 2 ) = 0 in I" then follows from the fact that

is a homeomorphism of U, onto an open set containing I". We can now write fl(zl) instead of f l ( z l ,z , ) and consider fi as a continuously differentiable mapping of El = KP into F; we then have f(x) = fl

(!r H(,f(,x-))) for

proves that y

x E U ; in other words, y

I

-+

=fl

- H ( y ) is a homeomorphism r

z1 -+fl(zl)the inverse homeomorphism.

(1. 1

- H ( y ) for y E ~ ( U )This . *

of f(U) orito Ip c El, and

280

X

EXISTENCE THEOREMS

Consider now K"' as the product El x E, with E, = K"-". Let T be the linear bijection of E, onto the supplement Q of P in F generated by cl,,,, . . . , d,,, , which maps the canonical basis of K'"-" onto dD+,,. . . , d,, . We define P ( Z ~ z,) , =,f,(z,) + T(z,) for z , E Ip, z 3 E it is obviously (8.9.1) n continuously differentiable mapping. By definition, we have H(P(z,,z 3 ) )= H(f,(z,)) = rz, ;hence the relation i ( z I , z,) = r ( z ; ,z ; ) implies z ; = z , , and then boils down to T(z,) = T(z;),which yields z3 = z ; ; therefore 2: is it?jectire. The relation S , = 0 proved above shows that for any z , E Ip,fS;(zl)= rL, where s is any point i n U such that f ( x ) =f,(z,); the derivative of I' at ( z , , 2 , ) is therefore the linear mapping ( t , , t,) + rL, . t , + T ( t 3 )((8.9.1) and (8.1.3)). But as the restriction of H to P, is injective, P, is a supplement of Q in F, hence r ' ( z l ,z,) is a litiear iiomeotiior~~hist~i of K"' onto F. For any point ( z , , z 3 ) E I"', there is therefore an open neighborhood W of that point in I" such that the restriction of to W is a homeomorphism of W onto an open subset r(W) of F, by (10.2.5). As in addition is injective, it is a homeomorphism of I"' onto the open subset V = r( I"'), whose inverse is continuously differentiable in V. The relation f = o f o I I then follows from the definitions. 11

11

11

0

(10.3.2) I f t l i e raiik of j ' ( a ) is equal to ti (resp. to n i ) , thn7 the coiiclusioti of (10.3.1) holds li'itiz p = n (resp. p = n i ) . Indeed, at the beginning of this section we have seen that there exists then a neighborhood of a in which the rank of ['(.Y) is > / I (resp. > t ? i ) , hence equal to ti (resp. to 171) since it is always at most equal to inf(in, t i ) (A.4.18).

PROBLEMS

1. Let E, F be two Banach spaces, A an open neighborhood of a point x o E E, f a continuously differentiable mapping of A into F. (a) Suppose f ' ( x o ) is a linear homeomorphism of E onlo its image in F; show that there exists a neighborhood U c A of .yo such that f is a homeomorphism of U onto f(U) (use Problem 3 of Section 10.1). (b) Supposef'(x,) is surjective; then there exists a number a .r0 having the following property: if Nisthekerneloff'(xo),for every s c E,one has 1 : f ' ( x O.)s I l 3 ( 1 . inf,It 1 .yii IS

N

(12.16.12). Show that thereexists a neighborhood V c A of xo such thatf(V) is a neighborhood o f f ( x o )in F (use Problem 8 of Section 10.1). 2. Let A be an open subset of Cp, and f a n analytic mapping of A into C". Show that if fis inrjecrioe, then the rank of Df(s) is equal t o p for every .Y F A. (Use contradiction, and induction on p ; for p = I, apply Rouche's theorem (9.17.3). Assume Df(o) has a rank < : p for some u t A ; show first that after performing a linear transformation in F, one may assume that, if f ( z ) ( f i ( z ) ,. . . , f p (z)), then D,/,(N)- 0, and if g ( z ) (f2(z),...,f, ( z ) ) , the rank of Dg(o) is exactly p - I; then there is a neighborhood ~

3 THE RANK THEOREM

281

U c A of a such that Dg(x) has rank p - 1 for x E U. Using the rank theorem (10.3.1), reduce the proof to the case in which n = O,X(z) zk for 2 < k < p.) Is the result still true when C is replaced by R ? 3. (a) Let A be a simply connected open subset of C, distinct from C, and let a, b be two distinct points of fr(A). (Appendix to Chapter IX, Problem 6 . ) There exists a complex-valued analytic function h in A such that ( h ( ~ )=) (z ~ - a ) / @- b) (Section 10.2,Problem 7); h is an analytic homeomorphism of A onto a simply connected open subset B of C (Problem 2 and (10.3.1)); furthermore, B n (-B) = 0, hence there are points of C exterior to B. (b) Deduce from (a) that there exists an analytic homeomorphism of A onto a simply connected open subset of C contained in the disc U : Iz/ < 1 , and containing 0. 4. (a) Let A be a simply connected open subset of C contained in the unit disk U : / z / < 1, containing 0, and let H be the set of all complex valued analytic functions g in A, such that g is an injecliue mapping of A into C, Ig(z)J < 1 , g(0) = 0 and g’(0) is a real number >O. For each compact subset L of A, the set H,. of the restrictions to L of the functions of H is relatively compact in Kc(L) (9.13.2). Show that the set of real numbers g’(0) (forg E H) is bounded (cf. proof of (9.13.1)); let be the 1.u.b. of that set. Show that there is a function go e H such that g,,(O) = h (use the result of Section 9.17, Problem 5). (b) Suppose g E H is such that g(A) # U, and let c E U -g(A). Replacing g by g1 defined by g,(z) = e-‘@g(ze‘@), one can assume, for a suitable choice of 8, that c is real and >O. There exists a function h which is analytic in A and such that 7

h W 2 = (c - g(z))/(l - CdZ)) and h(0) = 2 / c > 0 (same argument as in Problem 3(a)); show that the function g2 defined by

h(z) =

(dY-gz(z))/(l -dc92(z))

belongs to H, and that g;(O) > g’(0). (c) Conclude from (a) and (b) that the function go defined in (a) is an analytic homeoniorphism of A onto U; using Problem 3(b), this implies that for any simply connected open subset D of C, distinct from C, there is an analytic homeomorphism of D onto U (“Riemann’s conformal mapping theorem ”). 5. (a) Let f be a complex valued analytic function in the unit disk U: (z[< 1 such that f ( O ) = 1 and If(z)l < M in U; show that for ( z /< I/M, I f @ - I1 < Mlz( (apply Schwarz’s lemma (Section 9.5, Problem 7) to the function g(z)= M(f(z) - 1 )/(M2 -f(z))). (b) Let f be a complex valued analytic function in U such that f ( 0 ) = O,f’(O) = 1, if’(z)l < M in U ; show that for 1z/ < I/M, i f ( z ) - zI S MIz12/2 (apply (a) tof’). (c) Show that under the assumptions of (b), the restriction o f f t o the disk B(0; I/M) is an analytic homeomorphism of that disk onto an open subset containing the disk B(0; 1/2M) (apply Rouchk’s theorem ((9.17.3), using the result of (b)). (d) For any complex number a E U, let [ ( ( z )= ( z - n)/(Zz - I ) ; for any complex , Jg’(z)l(l - lzl’) = valued function f analytic in U, show that, if g(z) = = f ( i t ( z ) )then If’(u(z))l(l ~ U ( Z ) ( for ~ ) any z E U . (e) Show that there is a real number (“Bloch’s constant”) b > 1/32’3 having the following property: for any complex valued function f analytic in U and such that Y(0) = 1, there exists zo E U such that, if xo = f ( z o ) , the open disk B of center xo and radius h is contained in f ( U ) and there is a function g, analytic in B and such ~

282

X EXISTENCE THEOREMS

that g(B) c U and f(g(z)) = z for z E B. (Consider first the case in which f is analytic in a neighborhood of 0 , and take for zo a point where lf’(z)l(l - I z / ’ ) reaches its maximum; use then (d) to reduce the problem to the case in which zo = 0, and apply in that case the result of (c) to a function of the form a +f(Rz), where a and R are suitable complex numbers. In the general case consider the functionf((1 - E)z)/(~- E ) , where E > 0 is arbitrarily small.) 6. (a) Let 9)) be the set of all complex valued functions f analytic in the unit disk U: /zl < I , such that f ( U ) does not contain the points 0 and I . For any function f~ 91), there is a unique analytic function g in U such that exp(2n(q(z)) =f(z) in U and (.P(g(O))I< rr (Section 10.2, Problem 7); g(U) does not contain any positive or negative integer. Furthermore (same reference) there is an analytic function h in U such that g(z)/(g(z) - 1 ) = ((I h(z))/(l - h ( ~ ) ) ) h(U) ~ : does not contain any of the points 0, 1, and c : = (dn v’n(n integer > 1). Finally, there is an c; = (dnf .\/n-analytic function g, in U such that exp(y(z)) = h(z): y(U) does not contain any of the points log c; 2k7ri, log c t 2krri (k positive or negative integer, n > I). Show that no disk of radius > 4 can be contained in g,(U); using Problem 5(e), deduce from that result that Ig,’(x)l < 4 / N I - 1x1)

+

+

+

+

for 1x1 < I (consider the function t cy(x (I - I x / ) t ) , for a suitably chosen constant c). Conclude that there is a function F(if, v), finite and continuous in (C - (0, I } ) x [0, I[, such that for every function f~ 9X,loglf(z)/ < F(f(O), r ) for any IzI < r < 1 . (b) Let f~ 9) be such that either If(O)! < 1/2 or ! f ( O ) - I 1 < 1/2. Given r such that 0 < r < 1, show that either If(z)i < 5/2 for jz/ < Y, or there exists a point x such that 1x1 < Y and If(x)l > 1/2, l f ( x ) - 11 > 1/2 and Ilif(x)l 2 1/2. Applying the result of (a) to the function f((z - x)/(az - I)), conclude that there is a function F,(u, u), continuous and finite in [0, co[ x [0, 1[, such that for any function f~ 911, the relations If(0)I < s and JzJ< r imply If(z)l < Fl(s, r ) (“Schottky’s theorem”). 7. Let A be an open connected subset of C, and ( f , ) a sequence of functions of the set 911 (Problem 6 ) . Show that for any compact subset L of A, there exists a subsequence (fJ such that either that subsequence is uniformly convergent in L, or the sequence (I/f,,.) converges uniformly to 0 in L. (Using Schottky’s theorem, prove that the points x E A such that lim ( l / f , ( x ) )= 0 form an open and closed subset of A, hence equal to --f

+

“-1

m

A or empty: in the second case, show, using the compactness of L, that there is a subsequence of (6) which is bounded in a compact neighborhood of L, and apply (9.13.1); in the first case, use similarly (9.73.7) applied to the sequence (lif.).) 8. (a) Let f be a complex valued function, analytic in the open set V : 0 < / z - a1 < r , and suppose a is an essential singularity off(Section 9.15). Show that C - f ( V ) is empty or reduced to a single point (“Picard’s theorem”. Let W be the open subset of V defined by r / 2 < Iz - a1 < r and consider in W the family of analytic functions L(z) =f(z/2”); if there are at least two distinct points in C -f(V), apply Problem 7 to the sequence (fJ,and derive a contradiction with Problem 2 of Section 9.15, using (9.1 5.2).) (b) Deduce from (a) that if g is an entire function in C, which is not a constant, then C - y(C) is empty or reduced to a single point (consider g(l/z) in C - (0)). 9. (a) Show that there is an entire function f(x,y ) in C2 satisfying the identity

f(4x, 4Y) - 4 m Y ) = - S W X ,

+

- 2 ~ ) ) ~ 2 ~ ( 2 x -, 2 m

and such that the term of degree < I in the Taylor development o f f a t the point (0,O) are x y (Section 10.1, Problem 11).

+

4 DIFFERENTIAL EQUATIONS

283

(b) Let g ( x , y) = f ( 2 x , -2.~4, and let J(x, y ) = a(J g ) / a ( x ,y ) ; show that J(2x, -2y) = J ( x , y ) , and conclude that J ( x , y ) = -4 in C2 (express f ( x , y ) and g ( x , y ) in terms of f ( 2 x , -2y) and g ( 2 x , -2y)). Prove that the analytic mapping II : ( x , y) + ( f ( x , y ) , g ( x , y ) ) of C2 into itself is injective (if it was not, it would not be injective in a neighborhood of (0, 0), owing to the preceding expressions). (c) Show that there is a neighborhood of (1, 1) which is not contained in u(C2). (Observe that there exists E such that 0 < E < 1 and that the relations I f ( 2 x , - 2 y ) - l I < ~ , l g ( 2 x , - 2 ~ ) - - < ~ i m p l y J f ( x , y ) - I 1 < e a n d l g ( x , y ) - 11 < E ; conclude that the relations / f ( x ,y ) - 1 < F and lg(x, y) - I I < E would imply IfCO, 0) - 11 < E and (g(0,O)- 11 C F , a contradiction) (compare to Problem 8(b).)

4. DIFFERENTIAL E Q U A T I O N S

Let E be a Banach space, I an open set in the field K, H an open subset of E, f a continuously differentiable mapping of I x H into E. A differentiable mapping u of an open ball J c I into H is called a solution of the differential equation (10.4.1 )

x’ = f ( t , x )

if, for any t E J , we have (10.4.2) u’(t) = f ( t , u ( t ) ) . It follows at once from (10.4.2) that u is then continuously differentiable in J (hence analytic if K = C, by (9.10.1)). (10.4.3) In order that, in the ball J c I of center t o , the mapping u of J into H be a solution of (10.4.1 ) such that u(to) = xo E H, it is necessary and sufJjcient that u be continuous (resp. analytic) in J if K = R (resp. K = C), and such tllat

(10.4.4)

u ( t ) = xo

+

J:

f(s, u ( s ) ) ds

(where, if K = C, the integral is taken along the linear path 5 + to -t- c(t - to), 0 d 5 < 1). This follows from the definition of a primitive, for (when K = C) iff is continuously differentiable and u is analytic, then s + f ( s , u(s)) is analytic (9.1 0.1 ). (10.4.5) (Cauchy’s existence theorem) / f ’ f is continuously differentiable in I x H, for any to E I and any x o E H there exists an open ball J c I of center to such that there is in J a solution u of (10.4.1) such that u(t,) = x,, .

We first prove a lemma:

284

X

EXISTENCE THEOREMS

(10.4.5.1) Let A be a conipact iiietric space, F a metric space, B a coinpact subset of F, g a conti~iuousniappiiig of A x F into a metric space E. Tlieii there is a neighborhood V of B in F such that g(A x V) is hounded irz E.

For any t E A and any z E B, there is a ball S , , z of center t i n A and a ball UI,zofcenter z in F such thatg(S,,: x U , , z ) is bounded, sincey iscontinuous. For any z E B, cover A by a finite number of balls S , , , z and let V, be the ball Ur,,:of smallest radius. Then g(A x V,) is bounded (3.4.4). Cover now B by finitely many balls Vz,; the union V of the Vz, satisfies the requirements (3.4.4.).

(a) Suppose first K = R. Let J , be a compact ball of center to and radius a, contained in I . By (10.4.5.1) there is an open ball B of center xo and radius b, contained in H, and such that M = sup llf’(t, x l l and k =

sup

(f.xkJo x

( i , x k J c tx B

B

]lDz.f(t,x)l\ are finite. Let J, for r < a be the closed ball of

center to and radius r, and let F, be the space of continuous mappings y of J, into E, which is a Banach space for the norm Ilyll = sup Ily(t)ii ISJ,

Let V, be the open ball in F,, having center xo (identified to the constant mapping f -+ xo) and radius 6. For any y E V,, the mapping (7.2.1)

t

-+

xo

+ Ii: f ( s , y ( s ) )ds is defined

and continuous in J,, since

J(S)

E

B by

definition, for y E V,; let g ( y ) be that mapping; g is thus a mapping of V, into F,. We will prove that for r small enough,g verifies the conditions of (10.1.2); applying that theorem and (10.4.3) will then end the proof, with J = j,. Now, for any two points y l , in V,, we have, by (8.5.4) j q 2

Ilf’(s9 Y l W )

- . f h vz(s))II

1.

- IlYl(4 -

L’2(s)//

d k . IIYI - Yzll

for any s E J, ; therefore, by (8.7.7), for any t E J,,

hence llg(yl) - g(y2)I(d krlIy,

-

J ~ ~ I I . On

the other hand, for any

lljr:

J’ E

V,,

d M for any s E J,, hence f ( s , .As)) dsI1 d M r by (8.7.7) and therefore I/g(x,) - xo/Id Mr. We thus see that in order to be able to apply (10.1.2),we should have k r < I and Mr < b(I - kr), and both inequalities will be satisfied as soon as r < b/(M + kb). Ilf(s, y(s))II

(b) Suppose now K = C; define J , , J,, B, M , and k as above, and let F, be the space of mappings y of J, into E which are continuous it7 J, and analj~tic in j,. This is again a Banach space for the norm ((yll= suplly(t)ll, by (7.2.1) and (9.12.1). For y

E

V,, the mapping t + xo +

i E

J,

f ( s , y(s))cls again belongs

5

COMPARISON OF SOLUTIONS OF DIFFERENTIAL EQUATIONS

285

to F,, for it is analytic in 9, since s - t f ( s , y(s)) is (9.7.3); and its continuity in g of V, into F,, and the end of the proof is then unchanged.

J, at once follows from (8.11.1). We therefore have defined a mapping

(10.4.6) Remark. The proof of (10.4.5) shows that the result is still valid when K = R and when f satisfies the following weaker hypotheses: (a) for every continuous mapping t + w ( t ) of I into H, t + f ( t , w ( t ) ) is a regulated function in I (Section 7.6); (b) for any point ( t , x) E I x H, there is a ball J of center t in I and a ball B of center x in H such thatfis bounded in J x B, and there exists a constant k 2 0 (depending on J and B) such that IIf(s, Y , ) - f ( s , Y J I I ,< 4 y l - y,II for s E J, yl, y , in B. Such a function f is said to be locally lipschirzian in I x H ; equation (10.4.2) is then to be understood as holding only in the complement of an at most denumeruble subset of J. This last remark also enables one to replace the open intervals I and J by any kind of interval in R.

5. C O M P A R I S O N O F S O L U T I O N S O F D I F F E R E N T I A L E Q U A T I O N S

We say that a differentiable mapping u of an open ball J c I into H is an approximate solution of (10.4.1 ) with approximation E if we have llu’(t) - f ( f ,

for any t

u(t))ll

d&

E J.

(10.5.1) Suppose lID,f(t, x)ll d k it7 I x H. I f u, u are two approximate solutions of (10.4.1 ) in an open ball J of center t o , with approximations E ~ E, ~ then, for any t E J , we h m e

(For k = 0, (eklt-tol- l ) / k is to be replaced by It - tol).We immediately are reduced to the case K = R, to = 0 and t 2 0 by putting t = to a t , la1 = 1, t 2 0 ; then if ul(t) = u(to a t ) , 01(5) = v(t, + a t ) , u1 and v1 are approximate solutions of x’ = af(ro + at, a - l x ) , whence our assertion. From the relation ilu’(s) - f ( s , u(s))/I < in the interval 0 < s 9 t , we deduce by (8.7.7)

+

+

,

286

X

EXISTENCE THEOREMS

and similarly

l

whence

o(r)

- v(0) -

s:

1

f(s, u ( s ) ) d s = E2 t

From the assumption on D,fand from (8.5.4) and (8.7.7) this yields (10.5.1.2.)

w(t) d w(0)

+ + (El

E2)t

+k

J:

w(s) d s

where w ( t ) = llu(t) - v(t)(l. Theorem (10.5.1) is then a consequence of the following lemma:

u,

(10.5.1.3) in an inferual [0,c ] , cp and $ are two (Gronwall's lemma) regulated functions 2 0, thenf o r ar7y regulateLlfiinction MI 0 in [0,c] satisfying the inequality (10.5.1.4)

"(tj

< cp(t) +

we have in [0, c]

(10.5.1.5) Write y ( t ) ) =

s'

0

1:

4 0 G Y(t) + j'cp(s)$(s)

Ic/(s)w(s) ds

e x P ( [ s k ) di) ds.

0

$(s)it(s)

ds; y is continuous, and from (10.5.1.4) it follows

that, in the complement of a denumerable subset of [0, c ] , we have Y ' ( t ) - $ ( f ) Y ( f ) < cp(t)$(t)

(10.5.1.6)

by Section 8.7. Write z ( t ) = y ( t ) exp( - \' $(s) ds); relation (10.5.1.6) is -0

equivalent to

By (8.5.3) and using the fact that z(0) = 0, we get, for t E [0, c ]

whence by definition Y(t>

~ ; c p ( s l wexP(jsZ$(41d t )

and (10.5.1.5) now follows from the relation

ds

w ( t ) G cp(t)

+y(t).

5

COMPARISON OF SOLUTIONS OF DIFFERENTIAL EQUATIONS

287

(10.5.2) Suppose f is continuously diflerentiable in I x H . If u, c are tn’o solutions of (10.4.1), dejiied in an opeti ball J of center to and such that u(t,) = u(f,), then u = 2’ in J.

It is enough to prove that u and z‘ coincide in every compact ball L of center to contained in J. This follows from (10.5.1) applied to u and u, provided we know that D, f is bounded in some set L x H’, where H’ is an open subset of H containing both u(L) and z(L). But the existence of such a set follows at once from (10.4.5.1 ).

(10.5.3) Suppose E isjiiite dimensional a n d f i s analytic in I x H. Then any solution of (10.4.1) in an open hall J c I is analytic.

This is immediate by definition if K = C. Suppose K = R, and let E = R”; then for any point ( t o ,xo) E I x H there is a ball Lo c C of center to and a ball P c C” of center x, such that L, n R c I and P n R” c H, and an analytic mapping g of Lo x Pinto C” whose restriction to (Lo n R) x (P n R”) coincides with f (9.4.5). There is by (10.4.5) an open ball L c Lo of center to in C such that there exists a solution u of the differential equation z’ = g(t, z), taking the value xo at the point t o , and D is analytic in L. Using the relation u’(t) = g(t, c ( t ) ) , and the definition of g and it is immediately verified by induction on n that all derivatives d”)(t,) belong to R”; hence (9.3.5.1) u ( t ) belongs to R” for t E L n R. This proves that the restriction u of u to L n R is a solution of (10.4.1) (see Section 8.4, Remarks), such that u(to) = xo . But by (10.5.2),any solution M.’ of (10.4.1 ) in a ball M of center t , such that iv(to) = x o coincides with u in L n M , hence is analytic at the point t o . Q.E.D. 21,

(10.5.4) Remark. When K = R, the proof of (10.5.1) shows that the inequality (10.5.1 .I ) is still valid when f i s lipschitziaii in 1 x H for a constant k 3 0, i.e. such that condition (a) of (10.4.6) is satisfied and that ll,f(t, xl) - f ( t , x2)ll d k . llxl - x,I/ for any t E I, x,,x 2 in H ; J can then be taken as an interval of origin (or extremity) t o , containing t o , u and u are primitivesof regulatedfunctions in J, and the relations Ilu’(t) - f ( t , u(t))11 < cl, llu’(t) - f ( t , v(t))II < E, are only supposed to hold in the complement of an at most denumerable subset of J. The uniqueness result (10.5.2) holds likewise (when K = R) under the only assumption thatfis locally lipschitzian (10.4.6) in I x H, when one takes for J an interval having to as origin or extremity, and one only requires that the relations u‘(t) = f ( t , u ( f ) ) ,u ’ ( t ) = g(t, u(t)) hold in the complement of an at most denumerable subset of J.

288

X

EXISTENCE THEOREMS

(10.5.5) Let f be continuously differentiable in I x H i f K = C, locally 11)schitzian in 1 x H i f K = R. Suppose v is a solution of (10.4.1) defined in an open ball J: If - t o / < r, suclz that j c I , that tl(J)c H,and that t + f ( t , 4 t ) ) is bounded in J . Then there exists a ball J‘ : It - toI < r’ contained in I , with r‘ > r, and a solution of (10.4.1) dejined in J’ and coinciding with v in J . (a) K = R. By assumption, we have 11 f ( t , v(t))ll < M for t E J, hence ilu’(t)ll < M in the complement of an at most denumerable subset of J. This implies I ~ D ( S ) - c(t)il < MIS - tI for s, t in J by the mean value theorem (8.5.2). From the Cauchy convergence criterion (3.14.6) we conclude that the limits c((t0 - r ) + ) and u((t, + r ) - ) exist and belong to v(J) c H. By (10.4.6), there exists a solution )ixl (resp. w,) of x’ = f ( t , x ) defined in an open ball U, (resp. U,) of center t , + r (resp. t , - r ) , contained in I , and taking the value v ( ( t o + r ) - ) (resp. u((t, - r ) + ) ) at this point; from (10.5.4)it follows in addition that (resp. coincides with 11 in U, n J (resp. U, n J), and the proof is therefore concluded in that case. (One may observe that it has not been necessary to check the existence of derivatives on the left or on the right for v (extended by continuity) nor for w1 and w,, at the points to - r and to + r.) (b) K = C. For any complex number 4‘ such that l i l = I , put t = t, + is, with s > 0, and P,(s)= ~ ( t+, is). Then the same argument as in (a) proves that u,(r-) exists and is in H ; hence there exists a solution H!, of x’ = f ( t , x ) defined in an open ball V, of center t , + i r , contained in J, such that w,(I, + i r ) = u,(r-). From (10.5.4) it follows that itsi and u coincide in the intersection of J n V, with the segment of extremities t , and t , + ( r ; as these functions are analytic in J n V,, they coincide in J n V, by (9.4.4). Now cover the compact set ( t - t o [= r with finitely many balls Vcc( 1 < i < m ) ; if V,{ n VCj# @, the functions w,, and coincide in VC4n VCj, for both coincide with 11 in the nonempty open set J n V,, n VCj,and we have only to apply (9.4.2) (to show that the preceding intersection is not empty, remark that the assumption implies r l i i - ijl < p i + p i , where p i , pi are the radii of VCi and V,,; hence there is 2 ~ 1 0 I,[ such that rAlii - iil < p i and and r(l - A ) l i i - ijl < p i ; it follows that the point t, + r((l - A)(, Aij) belongs to J n V,, n Vij). There is therefore a solution of x’ = f ( t , x) equal to v in J, to wi, in each of the V l i ,and there is an open ball of center t , and radius Y’ > r contained in the union of these sets (3.17.1 1), which ends the proof. ~

3

~

)

+

(10.5.5.1) It follows from (10.5.5) that if r , is the 1.u.b. of all numbers r such that j c I a n d F ) c H, either ro = co,or, if J, is the open ball It - tol < y o , one of the two relations jo$ I , $ H holds.

+

zi(J,)

5

COMPARISON OF SOLUTIONS OF DIFFERENTIAL EQUATIONS

289

(10.5.6) Let f, g be two continuously diflerentiable mappings of I x H into E, and suppose that, in I x H , 11 f ( t , x) - g(t, x)II 6 c( and Ij D,g(t, x)I/ 6 k. Let ( t o , xo) be a point qf’ I x H , p, p two numbers > 0, and q($ = ehc - 1 Lieht (a p) -for 2 0. Let u be an approximate solution of x’ = g(t, x),

+ +

<

k with approximation 8, dejinedin an open ball J : It - t o [< b contained in 1, and such that u(to)= xo and for ar7y t E J, the closed ball of center u ( t ) and radius cp (It - t o [ )is contained in H. Then,for any y E H such that Ily - xoII 6 p there, exists a unique solution v of x’ = f ( t , x ) , de$ned in J, taking its values in H ar7d such fhat u(to) = y ; furthermore ilu(t) - V(t)II 6 q(lt - tol),for t E J.

Let A be the set of numbers r such that 0 < r 6 b and that there exists a solution u, of x’= f ( t , x ) with values in H, defined in the ball J, : It - tol < r and such that u,(to) = y . By Cauchy’s existence theorem (10.4.5), A is not empty. Moreover, we have, in J,, llv:(t) - g ( t , v,(t))Ij 6 z; in other words v, is an approximate solution of x’ = g(t, x ) with approximation a, and by (10.5.1.1) we conclude that llu(t) - v,(t)Il < cp(lt - t o l ) in J,. If r, r’ are in A and such that r < r‘, then v, and u,. coincide in J,, by (10.5.2) and (10.5.4). Let c be the 1.u.b. of A ; we have to prove c = b. Suppose the contrary; there is then a unique solution v of x’=f ( t , x) in J,, equal to v, in each of the balls J, with r < c, taking its values in H and such that iiu(t) - r(r)il d cp(lt - r o l ) in J,. We therefore have Ilg(t, L’(t)))II < Ilg(t, u(t))II kcp(If- tol) in J,, and as t ’ g ( t , u(t)) is continuous in J , , it is bounded in that compact ball; from which it follows that t + g ( t , v ( t ) ) is bounded in J,. On the other hand, any cluster point z of v(J,) is the limit of a sequence (u(t,)) where t , E J, and t, tends to to cc with 1“ d 1 ; by continuity, we have JIz- u ( t o ci)II 6 q(cICl), hence Z E H by assumption. We thus can apply (10.55) and obtain a solution of x’= f ’ ( t , x ) defined in a ball J,. with r‘ > c and taking the value y at t o , which contradicts the definition of c.

+

+

+

When K = R,one may, in the statement of (10.5.6), replace J by an open interval ]c, d [ containing the point t o . (10.5.6.1) We again remark that if K = R, we can relax in (10.5.6) the hypotheses on f and g, supposing merely that y is lipschitzian for the constant k, andflocally lipschitzian in I x H. PROBLEMS

1. Let f ( i ,x ) be a real valued continuous function defined in the set I t / < a, 1x1 < b in R2,such that f ( r , x ) < 0 for tx > 0, and f ( r , x ) z0 for t x < 0. Show that x = 0 is the unique solution of the differential equation x’ = f i t , x ) defined in a neighborhood

290

X

EXISTENCE THEOREMS

of 0 and such that x(0) = 0 (use contradiction, and consider, in a compact interval containing 0, the points where a solution reaches its maximum or minimum). 2. Let f ( t , x) be the real valued continuous function defined in R Z by the following conditicns: f ( t , x) = -2, for x > t 2 , f ( t , x) = -2x/t for 1x1 < t 2 , f ( t , x ) = 2 f for x < - t Z . Let (y,) be the sequence of functions defined by y o ( t ) = /’, y.(t)

=

J:

f ( u ,Y,-I(L~))L/u

for n > 1. Show that the sequence (y.(t)) is not convergent for any t # 0, although the differential equation x’ = f ( f , x ) has a unique solution such that x ( 0 ) 0 (Problem I). a, 3. For any pair of real numbers CL > 0, /3 > 0, the function equal to -(t - C L )f o~ r t -: to 0 for a < t < P, to (t - 8)’ for t > /3, is a solution of the differential equation x’ = 2 1 ~ I ”such ~ that x ( 0 ) = 0. Let uo be an arbitrary continuous function defined in a compact interval [a, / I ] , and define by induction

rr.(t) = 2

J’:

~ U - ~ ( . ds S ) for ~ ~ t~ E~ [a, 01.

Show that if y is the

largest number in [a, b] such that u O ( t )= 0 in [a, y ] , the sequence ( i d converges uniformly in [a, b] to the solution of x ’ = 2 1 ~ 1 ’ which /~ is equal to 0 for a < t < y , to ( t y ) 2 for y < r < h. (Consider first the case in which uo(t)= 0 for t < y , u o ( t ) k(t - y)2 for y < t < b. Next remark that, replacing if necessary uo by u l , one may suppose that uo is increasing in [a, b ] ; observe that for any number E > 0, there are two numbers k l > 0, k z > 0 such that in [a, b] -

4.

where vo(t)= 0 if t < 0, vo(t)= t 2 if t > 0.) The notations being those of Section 10.4, suppose K == R,fis continuous and bounded in I x H, and let M = sup llf(r, x)I/. Let x o be a point of H, S an open ball of ( ~ . x )xE HI

center xo and radius r, contained in H . (a) Suppose in additionfis uniformly continuous in I x S (a condition which is automatically satisfied if E is finite dimensional and I is contained in a compact interval I. such that f is continuous in l o x H). Prove that for any E > 0, and any compact interval [ t o ,to h] (resp.[to - h, t o ] ) contained in I and such that h < r/(M E ) , there exists in that interval an approximate solution of x’ =f ( t , x) with approximation E , taking the value xo for t = t o . (Suppose 6 > 0 is such that the relations Itl - f z I < 6, 11x1- xz I1 < 6 imply Ilf(t1, x l ) - ] ( I 2 , x2)l/ < E ; consider a subdivision of the interval [ t o , t o h] in intervals of length at most equal to inf(6, 6/M), and define the approximate solution on each successive subinterval, starting from to.) (b) Suppose E is finite dimensional and I = ]to - a, to a[. Prove that there exists a solution of x’ = f ( r , x ) , defined in the interval [ t o ,to c] (resp.[ro - c, t o ] )with c = inf(a, r/M), taking its values in S, and equal to xo for t = to . (“ Peano’s theorem ”: for each n, let u, be an approximate solution with approximation l/n, defined in J, = [ t o , t o c - ( l / n ) l ,whose existence is given by (a). Observe that for each m , the restrictions of the functions u. (for n 3 m ) to J,, form a relatively compact subset of the normed normed space KE(J,,,) (7.5.7), and use the “diagonal process” as in the proof of (9.13.2); finally apply (10.4.3)and (8.7.8).) 5. Letf’be the mapping of the space ( c o )of Banach (Section 5.3, Problem 5) into itself, such

+

+

+

+

+

+

that, for x

= ( x . ) , f ( x ) = (yJa

with y n =

+ -.n +1 l

Show that f is continuous in

(cd, but that there is no solution of the differential equation x’ - f ( x ) , defined in a

neighborhood of 0 in R, taking its values in (co), and equal to 0 for t = 0. (If there was

5

COMPARISON OF SOLUTIONS OF DIFFERENTIAL EQUATIONS

291

such a solution u ( t ) = ( u n ( f ) ) ,compute the value of each u,(t) by straightforward integration, and show that the sequence (u.(t)).to does not tend to 0 for t # 0.) (a) The notations being those of Section 10.4, let f be analytic in 1 x H if K = C, locally lipschitzian in 1 x H if K = R. Let lo be an open ball of center to and radius a, contained in I , and S an open ball of center xo and radius Y, contained in H. Let h(s, z ) be a continuous function defined in [0, a [ x [0, r [ c R2, such that h(s, z ) 2 0 and that, for every S E [0, a[, the function z+h(s, z ) is increasing in [0, Y[. Suppose that: ( I ) llf(t, x)l/ < h ( / t- t o / , /lx - xoll) in I. x S ; (2) there exists an interval [0, a ] with dc < a, and a function rp, which is a primitive of a regulated function T’ in [O, dc], and is such that ~ ( 0= ) 0, ~ ( s E) [0, r [ and ~ ’ ( s ) > h(s, ~ ( s ) ) in the interval [0, CL], with the exception of an at most denumerable set of values of s. Show that there is a solution I / of x’ = f ( t , x), defined in the open ball J of center to and radius a , taking its values in S and such that u(to)= xo ; furthermore, in J, llu(t) - xoIl < p(lt tot). (Use (10.5.5) to prove that there is a largest open ball Jo of center t o , contained in l o , and in which there is a solution u of x ’ = f ( t , x ) , taking its values in S and such that lIo(t) - X O / / < y(lt - t o l ) in J o , and furthermore that solution is unique; use then the mean value theorem to prove by contradiction that J c Jo .) (b) Suppose that H = E, and that there is a function h(z) > 0 defined, continuous and ~

increasing in [0, + a [ and , such that

Io+-

- = t m , and that

):

llf(t,

x)ll < h(llxll) in

l o x E. Show that every solution of x ’ = f ( t , x) defined in a neighborhood of t o , is defined in lo (use (a)). (c) If lif(t, x)ll < M in lo s S, then there exists a solution of x ‘ = f ( / , x) in the ball J of center to and radius inf(a, r/M), taking its values in S and such that u to)= x o (take h(s, z ) = M). Suppose K = E = C, and a 2 r / M ; show that, unlessfis a constant, there is an open ball J’ 3 in which u can be extended to a solution of x’ = f ( t , x) taking its values in S . (Observe that, due to the maximum principle ( 9 .5 .9 ) , Iic’(t)l < M for t E J; for any 5 such that 151 = I , consider the function I{&) = u(to 5s); arguing as in (10.5.5), prove that the assumptions of (10.5.5) are satisfied.) I t is not possible to take for the radius of J’ a number depending only on a, r and M , and not on fitself, as x)/2)”” (Section 9.5, Problem 8), with t = xo = 0, a = r = the examplef(t, x) = ((I M = I , shows (n arbitrary integer i I). Letfbe a real valued bounded continuous function in the open polydisk P: It - t o / < a, Ix- xoI < b in R2, and let M = sup I f ( f , x ) l ; let r = inf(a, b/M), and let I =

+

+

+

(1.X) E

P

]to- r , to i.[. Let (D be the set of all solutions I ( of x ’ = f ( t , x). defined in I, taking their values in the open interval ]xo- b, x o b[ and equal to xo for t = to ; the set CI, is not empty (Problem 4(b)). For each t E I , let u ( f , t o , xo) = inf u(t), w(t, t o ,xo) =

+

US@

sup u ( r ) ; show that u and w belong to CD (Section 7.5, Problem 1 I ) ; v(resp. w) iscalled the “ E O

minimal (resp. muximal) solutim of x’ = f ( t , x) in I , corresponding to the point ( t o ,XO). u ( t , t o , xo) in an interval For each T E I , let $. = U ( T , t o , xo). Show that u(t, T , of the form [ T , T h [ , if T 2 t o , of the form IT - h , T ] if T < to (with h > 0). Conclude that there is a largest open interval I t I ,f 2 [ contained in ]to- a , to a[ and containing t o , such that u(t, t o ,x,) can be extended to a continuous function g defined in ]/I, f 2 [ , taking its values in ] x o- b, xo -t /I[, and such that, for every t E PI, t 2 [ , g ( . s )= u(s, t , g ( t ) ) in an interval of the form [t,t h[ if t > t o , of the form It - h , t ] if t < to with h > 0). (Ifg, is another such extension of u ( t , t o , xo) in an interval It;, t i [ ,show that g andg, coincide in the intersection of ] I , , f 2 [ and It;, f ; [by considering the 1.u.b. (resp. g.1.b.) crf the points s in that intersection such that g and gt coincide in [ t o , s[ (resp. in Is, t o ] ) . Furthermore, either t l : to - a (resp. f 2 = to n ) , or g(tl +) = xo Jr b (resp.

6)

+

+

+

y(ti-)

= xo

i b).

+

292

X

EXISTENCE THEOREMS

(a) Generalize Gronwall’s lemma (10.5.1.3) to the inequalities

where v, # 1 , t+h2, 81, O2 are regulated functions 3 0. (Use induction on the number of integrals on the right-hand side.) (b) Let K(t, s) be a continuously differentiable function 3 0 defined in [0, c] x [0, c]. Suppose there are two regulated functions g, h, defined and 3 0 in [0, c ] , such that aK(t, s)/af< g(f)h(s).Show that the inequality w ( t ) < p(t) +lotK(f, s)w(s)ds

for a regulated function w

> 0 implies

w ( t ) G vl(t)

+ el(t)j)(s)w(s)

ds

where v1 and 81 are functions which one can explicitly compute when the functions

v,g and r ( t ) = K ( t , t) are known (consider the function y ( t ) = majorize its derivative). (c) Apply (a) or (b) to the inequality

w ( f )< t

+ h 2 f j : c A S w ( sds) +

fd

1:

K(t, s)w(s)ds and

w(s) ds,

where h > 0. Let w be a real function defined in an open interval I c R, and suppose wis the primitive of a regulated function w’ whose points of discontinuity are isolated in I, and such that in each of these points w’(t+) > w ’ ( t - ) ; suppose in addition that if E is the set of points of discontinuity of w’, the second derivative w” exists in I - E and w”(x) w(x) in I - E. (a) Show that if a, b are two points of I such that w(a) = w(b) = 0, then w(x) < 0 for a < x < b (use contradiction). Conclude that for any three points xl < x < x 2 in I one has w(xl) sinh(x2 - x) w(x2) sinh(x - xl) w(x) G sinh(x2 - xl)

+

(consider the difference w(x)- u(x), where u is a solution of the equation u”(x) - u(x) = 0 taking the same values as w at the points x, and x2).

6. LINEAR DIFFERENTIAL EQUATIONS

The existence theorem (10.4.5) can be improved in special cases: (1 0.6.1) Let I c K be an open ball of center to and radius r. Let j’be continuous in I x E ifK = R,continuously differentiable in I x E i f K = C,and such that

6 LINEAR DIFFERENTIAL EQUATIONS

293

Ilf(t, X I ) - A t , xJll < k(lt - t0I)llxl - x2II for t E 1, XI,x2 in E, where 5 --+ k(5) is a regulated function in [0,r[. Then for every xo E E , there exists a

unique solution u of (1 0.4.1), defined in I, and such that u(to) = xo .

We only have to prove that, if c is the 1.u.b. of the numbers p such that 0 < p < r and that there exists a solution of (10.4.1) defined in It - tot < p and taking the value xo at t o , then c = r (by (10.5.4)). Suppose the contrary; then, by (10.5.4), there is a solution v of (10.4.1) defined in J : It - tol < c and such that v(to) = xo . We are going to show that the conditions of (10.5.5) are satisfied; applying (1 0.5.5) then yields a contradiction and ends the proof. As here H = E, the condition v(J) c H is trivially verified, so we have only to check that t -+f ( t , v ( t ) ) is boundedin J . Now, in the compact interval [0, c], k is bounded and so is the continuous function t -+ 11 f ( t , xo)II in the compact x)ll < set 3; hence there exist two numbers m > 0, h > 0 such that ,?(fI[ mllxll h for t E J and x E E. This implies IIv’(t)ll < rnllv(t)ll h for t E J ; if we write w(5) = IIv(to + A()II with 111 = 1, the mean value theorem shows that

+

+

w(t) < llx0II + hc + rnJ; w(c) dc. We therefore can apply Gronwall’s lemma (10.5.1.3), which shows that Ilv(t)II < aemlr-tol+ b in J (a and b constants), hence u is bounded in J, and so is IIf ( t , v(t))ll < mllv(t)ll + h. (1 0.6.1. I ) Here again, when K = R,the condition of continuity on f can be relaxed to condition (a) of Remark (10.4.6).

A linear differential equation is an equation (10.4.1) of the special form

x’

(1 0.6.2)

= A(t) *

x

+ b(t)

(=f (t,x))

where A is a mapping of I into the Banach space 9 ( E ; E) of continuous linear mappings of E into itself (Section 5.7), and b a mapping of I into E. We have here H = E, and by (5.7.4)

Ilf(r,

x1) -

f(C

X2N

< IIA(t>ll

*

11x1

- x2II

for all t E I , x l , x 2 in E. Applying (10.6.1) and (10.6.1.1) we therefore get: (10.6.3) Let I c K be an open ball of center to . Suppose A and b are regulated in I if K = R, analytic in I if K = C. Then, for every xo E E , there exists a unique solution u of (10.6.2), defined in I and such that u(to) = xo .

Observe that if b = 0, and xo = 0, the solution u of (10.6.2) is equal to 0. From (10.6.3) we easily deduce the apparently more general result:

294

X

EXISTENCE THEOREMS

(10.6.4) The assumptions being the same as in (10.6.3), f o r every s E I and every xo E E, there is a unique solution u of (1 0.6.2) defined in 1 and such that u(s) = xo .

Replacing t by t - t o , we may assume that to = 0. Suppose I is a ball of radius r ; the mapping

t+r2-

t-s St - r2

is an analytic homeomorphism of 1 onto itself, mapping s on 0: indeed, one has t -s t'=r2-=it-1

hence, if It1

(

r_' I---- r2 - Is[') s r2 - St

< r, we have lr2 - it1 < r(r

+ Isl)

whence

our assertion follows from the fact that, conversely,

t=r2-

t' - s

St' - 1

.

Now, if

and

one sees at once that if v is the unique solution of the differential equation X' = A,(t)

defined in I and such that v(0)

.x + bl(t)

= xo, then

is the unique solution of (10.6.2), defined in 1 and such that u(s) = xo.

7 DEPENDENCE OF THE SOLUTION ON PARAMETERS

295

When E = K", A ( t ) = (aij(t))is an n x n matrix, b ( t ) = (b,(t))a vector, the a i j ( t ) and b,(t) being regulated in 1 if K = R , analytic if K = C; if

x = (xi)'
X; =

C aij(t)xj+ bi(t)

(1

j= 1

< i < n).

The (scalar) linear differential equations of order n > 1 (10.6.6)

D"x - a,(t)D"-'x - . . . - a,-,(t)Dx - a,(t)x = b(t)

are equivalent to special systems of type (10.6.5); one has only to write < p < n, and (10.6.6) is equivalent to

x1 = x, x p = D P - ' x for 2

X ~ = X ~ +for ~

(10.6.7)

x;

= al(t)x,,

1
1

+ a,(t)x,-, + - - .+ a,,(t)xl + b(t).

7. D E P E N D E N C E OF T H E S O L U T I O N O N PARAMETERS

(10.7.1) Let E be a Banach space over K, I an open subset of K, H an open subset of E, P a metric space, f a mapping of I x H x P into E. Suppose that: (1) for any z E P, ( t , x ) -+ f ( t , x, z ) is a continuously differentiable mapping of I x H into E; ( 2 ) f and D,f are continuous in I x H x P. Then, for any point ( t o ,xo , zo) E I x H x P, there exists an open ball J c I of center t o and an open ball T c P of center zo such that, for each z E T, there exists in J one and only one solution t + u(t, z ) of the equation x' =f ( t , x, z ) such that u(to,z ) = xo . Moreover the mapping ( t , z ) + u(t, z) is bounded and continuous in J x T.

The proof is very similar to that of (10.4.5). Let J, be a compact ball of center to and radius a contained in I. By (10.4.5.1), there is an open ball B of center xo and radius b contained in H, and an open ball T of center zo in P, such that 11 f ( t , x,z)ll d M and IID, f ( t , x, z)ll < k in J, x B x T. For r < a, let J, be the closed ball of center to and radius r. If K = R, we define F, to be the space of bounded continuous mappings y of J, x T into E, which is a Banach space. If K = C, we define F, as the space of mappings y of J, x T into E which are bounded and continuous in J, and such that, for any z E T, t -+ y ( t , z ) is analytic in i,; this is again a Banach space by (9.12.1). The remainder of the proof of (10.4.5) is then unchanged. For linear differential equations, there is a better result:

296

X

EXISTENCE THEOREMS

(10.7.2) Let I c K be an open ball of center to; suppose A and b continuous in I x P, and, i f K = C, such that for each z E P, t -+ A(t, z ) and t -+ b(t, z ) are analyticin1. ForanyxEE,let t -+u(t,z)bethesolutionofx' = A ( t , z ) * x b(t,z) dejned in I and such that u(to,z ) = x,; then u is continuous in I x P.

+

Let zo E P, and consider an arbitrary compact ball J c I of center to and radius r ; it will be enough to prove that u is continuous at each point (t,zo)where t E J. As t+ u(t,zo)is continuous in J, it is bounded in that compact set, let IIu(t, zo)II < M in J. By (10.4.5.1),there is a neighborhood U of zo in P such that IIA(t, z)ll < k for z E U and t E J. Given arbitrarily an E > 0, let us show next that there exists a neighborhood V c U of zo in P such that IIA(t, z ) - A(t, zo)[I< E and Ilb(t, z ) - b(t, zo)(I < E for t E J and z E V. We only have to remark that for any s E J there is a neighborhood W, of s in J and a neighborhood V, c U of zo in P such that the preceding inequalities holdin W, x V,; then wecoverJ by afinite number of neighborhoods WSi,and take for V the intersection of the V S i We . can now write d ( t , Z ) - u'(t, zo) = A(t, Z )

*

(u(t, Z) - u(t, ~

hence, for t E J and z

+ (A(t,Z ) - A(t, ~ 0 ) )U(t, zo) + b(t, Z ) - b(t, zo)

0 ) )

*

EV

< k . Ilu(t, Z ) - u(t, zo)II + E(M+ 1). Put t = to + 25 with 121 = 1,0 < 5 < r, and 4 5 ) = llu(t0 + At, z>- u(t0 + At, z0)Il; then by the mean value theorem, we have w(5) < E(M + 1)r + k w ( ( ) d( for 0 < 5 < r, and using (10.5.1.3), we obtain w(5) < E(M + ])rek' for 0 < 5 6 r, in other words, we have Ilu(t, z) - u(r, zo)II < E(M + l)rek' for IlU'(t, Z ) - U'(t, z0)II

1;

t E J and z E V; as continuous in J).

E

is arbitrary, this ends the proof (since t -+ u(t, zo) is

(1 0.7.3) In addition to the assumptions of (1 0.7.1), suppose that P is an open subset of a Banach space G, and that (1) ifK = R,f is continuously differentiable in I x H x P; (2) ifK = C, f is twice continuously differentiable in I x H x P (when E and G are finite dimensional, this is equivalent to saying that f is analytic in I x H x P (9.1 0)).Let J, c I be an open ball of center toand T, c P an open ball of center zo such that,for every z E TI, there is a solution t -+ u(t, z ) of x' =f ( t , x, z ) (necessarily unique by (1 0.5.2)) dejned in J, and such that u(to, z ) = xo . Then, for any open ball J of center t o , such that c J,, there

7 DEPENDENCE OF THE SOLUTION ON PARAMETERS

297

exists an open ball T c TI of center zo such that ( t , z) -+ u(t, z ) is continuously differentiable in J x T . Furthermore, for any z E T , t D2 u(t, z ) is equal in J to the solution U ( t ,z) of the linear differential equation -+

U'

(10.7.3.1)

= A(t, Z)

0

U

+ B(t,

such that U ( t o ,z) = 0, where A(t, z) D 3 f ( t , u(t, z),z).

Z)

= D,f(r,

u(t, z), z) and B(t, z) =

Let J be an open ball of center to and radius r, such that S c J,. By (10.4.5.1), there is an open ball S c H of center xo and an open ball T c T, of center zo such that D2f and D 3 f are bounded in J x S x T, let IIDzf(t,x , z)II < a and IID3f(t,x,z)ll < b. Then, by (8.5.2) and (8.9.1), we

have,

for t E J, x,,x2 in S , z,, z, in T. Taking (10.7.3.2) into account, we see that, by (10.5.1), we have, for t E J and zl, z, in T (10.7.3.3)

IIu(t, Zl) - u(t, zz)ll

< C l l Z l - ZZII

with c = b(e" - l)/a. We next prove that, given a point z E T and E > 0, there exists p > 0 such that, for any w E P such that z w E T and llwll < p , and any t E 5, we have

+

Ilf(t, u(t9 z

+ 4,z +

- f ( t , u(t, 4, 4

- A(t, z) * (u(t,z + w) - u(r, z))

(10.7.3.4)

- B(t,z ) *

wII

G E IIwII. Indeed, using (8.6.2), (8.9.1), the continuity of D , f and D3 fi n I x H x P, and relation (10.7.3.3), for any s E J, there is a neighborhood W, of s in J, and a number p(s) > 0 such that relation (10.7.3.4) holds for t E W, and llwll < p(s); covering 5 by finitely many W,:, we need only take for p the smallest of the p(si) to have (10.7.3.4). Due to the definition of u(t, z), (10.7.3.4) can also be written (10.7.3.5)

IID,u(t, z

+ W ) - Dlu(t,

G EIIWII.

Z)

- .4(t, Z)

*

(u(t,z

+ W ) - u(t, z)) - B(t, Z ) . wII

298

X

EXISTENCE THEOREMS

Now the existence of U ( t , z ) in J x T is guaranteed by (10.6.3), since D 2 f and D3f are continuously differentiable when K = C. Put v(t, z , M’)= u(t, z HI)- u(t, z ) - U(t,z ) . it’; this function has a derivative with respect to t equal to

+

D,o(t,

Z,

IV) = D,u(r, z

+ w) - D,u(t, Z ) - A(?,

Z)

. ( U ( t ,Z )

W)

-

B(t, Z) . W ,

by (10.7.3.1 ). Relation (10.7.3.5) therefore can be written IID,u(t, Z ,

IV)

- A(t, Z)

. ~ ’ ( tZ,,

n1)Ild

E lllt’ll,

for any t E J and any $11 such that z + I(’ E T and llwxll < p . In other words, v(t, z , it’) is an approximate solution, with approximation E 11 ~ 1 1 , of the linear differential equation y’ = A ( t , z ) . y.

(1 0.7.3.6)

Furthermore, we have u ( t o , z , cv) = 0 by definition; as jlA(t, z)II < a in J x T, we conclude from (10.5.1) (since 0 is a solution of (10.7.3.6)) that

II4t,

7-7

M’III

< COE I I 4

where co = (ear- l)/a, this inequality being valid for any t E J and any cv such that z 11:E T and llct~ll d p . As E is arbitrary, the definition of the derivative of a function shows that u is differentiable with respect to z at any point ( t , z ) E J x T and that D , u(t, z ) = U(t,z). Finally, from the assumptions and (10.7.2), it follows that U is continuous i n J x T ; on the other hand, D,u(t, z ) =.f’(t,u(t, z ) , z ) is continuous in J x T by (10.7.1). Therefore, by (8.9.1), u is continuously differentiable in J x T, and this ends the proof of (10.7.3).

+

(10.7.4) Suppose K = R, and f is p times continuously diferentiable (resp. E and G are finite dimensional a n d f is indefinitely diferentiable) in I x H x P. Then,for each open interual J of center to suclz that J c J,, it is possible to take T such that u is p times continuously diferentiable (resp. indefinitely diferentiable) in J x T. If p = 1, this is (10.7.3). Using induction on p , suppose we have proved the result for ( p - 1) times continuously differentiable mappings. Then, in the right-hand side of (10.7.3.1), A and B are p - 1 times continuously differentiable mappings in J x T (by (8.12.10)); therefore, by (10.7.3) (applied to U(t, z ) ) , D , u(t, z ) is p - 1 times continuously differentiable in J x T (when T has been conveniently chosen). On the other hand D,u(t, z ) = f ( t , u(t, z ) , z ) is also p - 1 times continuously differentiable in J x T by the induction hypothesis and (8.12.10); therefore Du(t, z) is p - I times continuously differentiable in J x T by (8.9.1), (8.12.9), and (8.12.10); but this

7 DEPENDENCE OF THE SOLUTION ON PARAMETERS

299

implies that u isp times continuously differentiable in J x T, by (8.1 2.5). When E and G are finite dimensional a n d f i s indefinitely differentiable, it follows from (3.1 7.1 0) that in the preceding argument one may choose T independently o f p , since all derivatives o f f are continuous; therefore u is indefinitely differentiable in J x T. Observe that in (10.7.4), one may replace J, by any open interval containing the point t o , and J by any open interval containing to and such that J c J,.

(10.7.5) Suppose that the Banach spaces E and G areJinite dimensional and that f is analytic in I x H x P. Then,for any open ball J of center t o , such that 5 c J,, it is possible to take T such that u is analytic in J x T. If K = C,this follows immediately from (10.7.1), (10.7.3), (9.10.1), and (9.9.4). If K = R, we apply an argument exactly similar to that of (10.5.3), which we accordingly suppress.

(10.7.6) Remarks. There are several improvements and variants of the preceding theorems. For instance, in (10.7.3), when K = R, the existence of D,fis not required to insure that D, u(t, z ) exists: we need only the continuity of D2,f and D3J’ as functions of (x,z), and their boundedness in J x S x T, as well as the fact that t + f ( t , h(t),z ) is regulated in I for any function 12 continuous in I and similarly for D,fand D,f:

PROBLEMS

1. The notations being those of Section 10.4, let I be an open ball in K of center to and radius a , S an open ball in E of center xo and radius Y, G the normed space Vz(I x S) (Section 7.2). For each M > 0, let G , be the ball l l f l l < M in G . Let L be the subset of G consisting of all continuous lipschifzian mappings of I x S into E (10.5.4); for each M > 0, let J, be the open ball of center to and radius inf(a, r/M); for each function f~ L n G , , there is a unique solution LI = U(f) of x’ = f ( t , x ) taking its values in S, defined in J, and such that u(to)= x o (Section 10.5, Problem 6(c)). (a) Let (A,) be a sequence of functions belonging to L n G , , and supposef, converges uniformly in 1 x S to a functionf; show that in the space %Z(J,), every cluster value of the sequence of functions u, = U(f.) is a solution of x’ = f ( f , x ) , taking its values in S, and equal to xo for 1 - to (use (10.4.3) and (8.7.8)).Give an example in which the sequence (/In) has 110 cluster value in VZ(JM)(see Section 10.5, Problem 5).

300

X

EXISTENCE THEOREMS

(b) Suppose in addition that E is finite dimensional; using the result of (a), give a new proof of Peano’s theorem (Section 10.5, Problem 4(b); use Ascoli’s theorem (7.5.7) and the Weierstrass approximation theorem (7.4.1 )). 2. (a) In the polydisk P: It - tol < a , Ix - xol < b in RZ,let g, h be two real valued continuous functions such that g ( f , x ) < h(t, x ) in P. Let u (resp. u ) be a solution of x’ = g ( t , x ) (resp. x’ = h(t, x ) ) defined in an interval [ t o , to c [ , taking its values in ]xo - b, xo b[ and such that u(to) = xo (resp. u(to) = x o ) ; show that u(t) < u ( t ) for to < t < to c (consider the 1.u.b. of the points s in [ t o ,to c[ such that u ( t ) < u ( t ) for to < t < s). (b) Let g be continuous and real valued in P, and let u be the maximal solution of x’ = g(t x ) corresponding to ( t o ,x o ) (Section 10.5, Problem 7); suppose u is defined (at least,) in an interval [ t o , to c[ and takes its values in ] x o - b, xo b[. Show that in every compact interval [ t o , to d ] contained in [ t o , to c [ , the maximal and minimal solutions of x’ = g(t, x ) E are defined and take their values in ]xo - b, xo b[ as soon as E > 0 is small enough, and converge uniformly to u when E tends to 0. (Given E~ > 0, there exists an s > to such that the maximal and minimal solutions of all theequations x’ = g(t, x ) E for 0 < E < eo, corresponding to ( t o , xo), are defined and take their values in ] x o - b, xo b[ fort in [to , s ] ; observe that all these functions form an equicontinuous set in [ t o ,s], and prove the uniform convergence to u in [ t o ,s ] by applying the result of (a), Ascoli’s theorem (7.5.7), (10.4.3), and (8.7.8). Finally, show that the 1.u.b. of the numbers d having the stated property is necessarily equal to c, using in particular the last statement of Section 10.5, Problem 7.) (c) In the polydisk P, let g and h be two continuous real valued functions such that g ( t , x ) < h(t, x ) in P. Let [ t o ,to c] be an interval in which a solution u of x’ = g(t, x ) such that u(to)= xo , and the maximal solution u of x‘ = h(t, x ) corresponding to ( t o ,xo) are defined and take their values in ]xo - b, xo b[. Show that u ( t ) < u ( t ) for to < I < to c (apply (a) and (b)). 3. (a) Show that the conclusions of Problem 6(a) of Section 10.5 are still valid when E is finite dimensional, and the assumptions are modified as follows: (1) f is supposed to be continuous in I x H (when K = R), but not necessarily locally lipschitzian; (2) 9 is the maximal solution (Section 10.5, Problem 7) of the equation z’ = h(s, z ) in [0, a ] , corresponding to the point (0,O). (Use the results of Problems l(a) and 2(b), and apply the diagonal process as in Problem 4(b) of Section 10.5.) (b) Suppose in addition that there exists a sequence (Yn)n30 of real valued functions, continuous in [0, a ] , taking their values in [ O , r ] , such that for n 3 1, Y , ( s ) =

+

+ +

+

+ + +

+

+

+

+

+

+

+

+

/:h([, Yn-l([))d[ for 0 < s < a. Let yo be continuous in J if K = R, analytic in J

if K = C, with values in S, and such that Ilyo(t)- x o I / < Yo(lt- tol) in J. Show that there exists a sequence (yn)nbl of mappings of J into S, which are continuous if

K = R, analytic if K = C, and such that y.(t)= x 0 +Joke, yn-l(0)) do, and that IIy,,(t) - xoII < Y,(lt - t o / )in J for every n 3 1. When K = C, conclude that the sequence (y,) converges in J (uniformly in every compact subset of J) to the unique solution u of x’ = f ( t , x). (Use (9.13.2) and the proof of (10.4.5).) Is this last statement still true when K = R andf is not supposed to be locally lipschitzian (cf. Section 10.5, Problem 2)? 4. (a) Let I = [ t o , to + c [ c R, and let w be a real valued continuous function 3 0, defined in I x R. Let S be an open ball of center x o in E, and let f be a continuous mapping of I x S into E such that for t E I, x 1 E S and x 2 E S, llf(t,xl) - f ( t , x 2 ) I I < w ( t , llxl - x211).Letu,ube twosolutionsofx’=f(t,x),definedin

7 DEPENDENCE OF THE SOLUTION ON PARAMETERS

301

I, taking their values in S, and such that

u(to) = x l , v ( t , x o ) = xz; let w be the maximal solution (Section 10.5, Problem 7) of z’ = w ( t , z ) corresponding to ( t o , llxl - xz I!), and suppose w is defined in I; show that in I, Ilu(t) - u ( t ) I/ < w(t). (For small E > 0, consider the maximal solution w(t, E ) of z’ = w ( t , z ) E corresponding to ( l o , 11x1- xz II), which is defined in [ t o , to d] if d < c, as soon as E is small enough (Problem 2); show that for to < t < to d, Ilu(t) - v(t)ll < w(t, E), using contradiction: consider the g.1.b. tlof the points t such that Ily(t)II > w(t, E), where y ( t ) = u(r) - v ( t ) , and observe that for t > t l

+

Ilu(t)ll- IlY(tl)ll <

+

+

IlY(t) - Y(t1)ll


(b) Let I’ = ]to - c, to], and suppose that the assumptions of (a) are verified when I is replaced throughout by 1’. Let now w be the minimal solution of z’ = w(t, z ) corresponding to (to ,IIxl - x2jl), and suppose it is defined in 1’; show that in 1’, Ilu(t) - u ( t ) I1 3 w(r) (same method). 5. (a) Let I be the open interval ]O,a[ in R, and let w be a continuous function in [0, +a[, such that w ( t , z ) 2 0, and w ( t , 0) = 0 for t E I; w can be extended to I x R by the condition w ( t , - z ) = w ( t , z) for z < 0. We suppose that if w is a solution of z’ = w ( t , z)defined in an open interval 10,a[ C I, such that w can be extended by continuity to the half-open interval [0, a[ by taking w(0) = 0, and that in addition w’(0) is then defined and equal to 0, then necessarily w ( r ) = 0 identically in 10, a[. Let now S be an open ball of center xo in a real Banach space E , f a continuous mapping of [0, a[ x S into E, such that, for 0 < t < a and xl, x 2 in S, llf(t, XI) - f ( t , xz)II S w ( t , 11x1- XZII). Show that in an interval [0, a]with a < a, there is at most one solution u of x’ = f ( t , x ) such that u(0) = x,, . (Use contradiction: if u is a second solution such that v(0) = x o , minorize Ilu(t) - v ( t ) Ij in 10,a],using Problem 4(b).) (b) Let @ t ) be a continuous function defined in 10,a[ and such that B(r) 3 0. Show that if the integralJ:w

+B w Z ;

dt is convergent, the result of (a)appliesto w(t,z) = 1

t

t

if, on the contrary, Jo‘*dt f

=

+a,give an example of a continuous real valued

function f i n [0, a[ x R,such that

and that the equation x’ = f ( t , x ) has an infinity of solutions in [0, a [ , equal to 0 for t = 0. (Let

+

definef(r, x ) as equal to (1 O(t))x/r for 1x1 < v(t),and independent of x for 1x1 3 TO).) 6. Let I be an open interval in R, H an open subset of a Banach space E over R. Let t o be a point of I. (a) Supposefis continuous in I x H, and that there is a number k such that 0 < k < 1 and that, for any t # t o , and xl, x 2 arbitrary in H,

X

302

EXISTENCE THEOREMS

There is then at most one solution of x' = f ( f , x ) taking a given value xo E H for t = to and defined in a neighborhood of to (Problem 5(a)). But in addition, if u , u are two approximate solutions of x' = f ( t , x) in an open ball J of center t o , contained in I, with approximations E ~ E . ~ and , such that u(to) = u(to) = xo, then, for any t E J

(Use the same method as in (10.5.1).) be the set of all real valued functions f, (b) Let I = 1- 1, I[, H = E = R, and let continuous in 1 x H, and such that, for t # 0 in I, if(/, X I ) - f ( t , x J < 1x1- xzl/ltl. There is then at most one solution of x' = f ( t , x) taking a given value for t = to and defined in a neighborhood of to (Problem 5(a)). But prove that there is no function ~ ( tE ), > 0 such that, for any pair ( u , u ) of approximate solutions, with approximation E , of any equation x' = f ( t , x) with f E a), such that u and u are defined in I and u(to) = u(to), the relation lIrr(r) - u ( t ) l / < ~ ( l t /E ), would hold for every t E I. (For any a E 10, 1 [, letf be the continuous function equal to x / t for 1x1 < t2/(a- t ) , 0 < t < a , and for t > a , and independent of x for the other values of ( t , x) such that t > 0; define f ( t , x) = f ( - t , x ) for t < 0. Take a : 0; let u ( t ) = E t for It/ < a , and take for u a solution of x' = f ( t ,x) E for the other values oft.) 7. The notations being those of Section 10.4, suppose E is finite dimensional and f is continuous in I x H; let ( t o ,x o ) be a point of I x H, J an open ball of center to contained in I, S an open ball of center xo such that S c H. Supposefis bounded in J x S, and the following conditions are verified: ( I ) There is af most one solution of x' = f(t, x) defined in an open interval contained in J and containing t o , and taking the value xo for t = t o . (2) There exists a sequence ( u , ) , ~of~continuous mappings of J into S such that

+

u.(t)

= xo

+Itr

f(s, r i n - l ( . s ) ) d.s

for n

>1

and

t E J.

0

+

(3) For every t E J , u " + ~ ( / )- u,(t) converges to 0 when n tends to m. Show that in every compact interval J' c J containing to the sequence(u,)converges uniformly to a solution of x' = f(/, x) equal to xo for t = t o . (Observe that the sequence (u.) is equicontinuous; use Ascoli's theorem (7.5.7), as well as (3.16.4) and (8.7.8).) 8. Suppose E is finite dimensional, w and f verify the conditions of Problem 5(a), and in addition, for every t E 10, a [ , the function z+ w(t, z ) is increasing in [0, +a[. There is then at most one solution of x' = f(t, x) defined in an interval [0, a [ c [0, a[ and taking the value xo for t = 0 (Problem 5(a)). Suppose in addition that there exists, in an interval J = [0, a ] C [0, a[, a sequence (u,Jnb0 of continuous mappings of J into S such that I&)

= xo

+s,'f(s,

U " - ~ ( S ) ) ds

(a) For every t E J, let y,(t)

=

for n > 1 and

l l ~ , , + ~( t u,,(t)/I, ) z.(t)

=

t E J.

supy,+,(t), and w ( t ) = k 2 0

inf z.(t). Show that the functions z, and w are continuous in J (use Problem 1 1 of XbO

Section 7.5). (b) Let t , t - h be two points of J (h > 0); show that, for every 6 > 0, there is an N such that, for n > N, lyn(t)- y.(t

- h)/ <

I' I-h

w(s, w(s)

+ 8) ds.

8 DEPENDENCE OF THE SOLUTION ON INITIAL CONDITIONS

303

(Use the mean value theorem (8.5.1), as well as ( 7 . 5 3 . ) (c) Deduce from (b) that, for n N

(consider in succession the cases z,(t) I w ( l ) - w(t - h)l

<

< z,(t

1'

- h)

W(S,

and z.(t)

w(s)) ds

> z,(t - h)). Hence

(by (8.7.8)).

t-h

(d) Conclude that w(t) = 0 in J (same argument as in Problems 4(b) and 5(a)), and using Problem 7, prove that the sequence (u,) converges uniformly in J to a solution of x = f ( t , x ) taking the value xo for t = 0. 9. The notations being those of Section 10.4, suppose E is finite dimensional, and f is continuous and bounded in I x H. Suppose in addition there is at most one solution of x' = f ( t , x) defined in any open interval J C I containing t o , and equal to xo E H for t = t o . Suppose that, for any integer n > 0, there exists an approximate solution it. of x' = f ( t , x), with approximation I/n, defined in 1 and taking its values in H, and such that u,(to) = xo . Show that in any compact interval contained inI, thesequence ( i t n ) is uniformly convergent to a solution u of x' = f ( t , x), taking its values in H and such that u(to)= x o . (Use the same argument as in Problem 7.)

8. DEPENDENCE OF THE SOLUTION O N INITIAL CONDITIONS

(10.8.1) Let f b e locally lipschitzian (10.5.4) in I x H if K I x H if K = C.Then,for any point (a, b) E I x H :

= R, analytic

in

(a) There is an open ball J cl of center a and an open ball V c H of center b such that,,for every point ( t o ,x,) E J x V, there exists a unique solution t u(t, t o , x,) of (10.4.1) defined in J, taking its values in H and such that d t , to xo) = xo * (b) The mapping ( t , t o , x,) + u(t, t o , x,) is uniformly continuous in JxJxV. (c) There is an open ball W c V of center b such that, for any point ( t , t o , x,) E J x J x W, the equation xo = u(tO,t , x ) has a unique solution x = u(t, t o , x,) in V. --f

9

9

(a) By assumption, there is a ball J, c I of center a and a ball B, c H of center b and radius r such that in J, x B, , 11 f ( t , x)ll Q M, and 11 f ( t , xl) f ( t , x2)l/ < k * I/xl - x211 for tE J,, x l , x 2 in B,, By (10.4.5) and (10.5.2) there is an open ball J, c J, of center I,, and a unique solution v of (10.4.1) defined in J,, taking its values in H and such that v(a) = b. We are going to see that the open ball V of center b and radius 4 2 , and the open ball J of center a and radius p , answer our specifications as soon as p is small enough. Apply

304

X

EXISTENCE THEOREMS

(10.5.6) to the case c1 = = 0; this shows that there exists a solution of (10.4.1) defined in J, with values in Bo , taking' the value xo E V at the point t o E J,

provided we have IIu(t) - 611

(1 0.8.1 .I)

for

every

IIv(t)

t~ J.

But

+ IIu(to) - xollekl'-'ol < r

by

the

mean

value

theorem,

we

have

< M It - a1 < M p forevery t E J ; as by assumption llxo - bll < r / 2 ,

- bll

the inequality (10.8.1 . I ) will be satisfied if p is such that (10.8.1.12)

(M p + -I)e 2 k p < r

Mp+

which certainly will be satisfied for small values of p > 0, since the left-hand side of (1 0.8.1.2) tends to 1-12when p tends to 0. (b) From the mean value theorem, we have (10.8.1.3)

for

to,t,,

(10.8.1.4)

- u(t2

IIu(t1, t o 9 xo)

t o 9 x0)II

9

dM

It2

- 1,

I

t , in J, xo in V. By (10.5.1), we have IIu(t, t o 9 XI)

- u(t, t o

9

x2)ll

< e2kp1x2 - x1 I

for t , t o in J, x , , x2 in V. Finally, (10.8.1.3) for Ilu(t1,

t2

9

xo) - xoll


to = t2

It2

yields by definition

- tl I

and as t + u(t, t , , xo) is the unique solution of (10.4.1) in J which is equal to u(t,, t 2 , xo) at the point t,, we have, by (1 0.5.1) (10.8.1.5)

IIu(r, t , , xo) - u(t9 t 2 , x0)II

< M e 2 k pI f 2 - [,I

for t , t , , t , in J, xo E V . The three inequalities (10.8.1.3), (10.8.1.4), and (10.8.1.5) prove that u is uniformly continuous in J x J x V. (c) By (10.8.1.3), we have IIu(t, t o , xo) - xoII < Mlt - tol < 2 M p in J x J x V. Suppose p satisfies (10.8.1.2) and in addition the inequality 2 M p < r/4 ; then, if W is the open ball of center b and radius 1-14,we have u(t, t o , xo) E V for t , t o in J and xo E W. Let x = u(t, t o , xo)for such values of t , t o , x,; then s -,u(s, t , x ) is defined in J and is the unique solution of (1 0.4.1 ) with values in H which takes the value x at the point t ; but as s + u(s, t o ,xo) has these properties, we have u(s, t , x ) = u(s, t o ,xo) for s E J; in particular xo = u(to, t o , xo) = u(to, I, x ) . Suppose nowy E V is such that u ( t o , t, y ) = x,; then s + u(s, t , y ) is a solution of (10.4.1 ) defined in J and taking the value xo for s = t o ; therefore u(s, t , y) = u(s, t o , xo) for any s E J, and in particular, for s = t , y = u(t, t o , xo) = x , which ends the proof.

8 DEPENDENCE OF THE SOLUTION ON INITIAL CONDITIONS

305

(10.8.2) With the notations of (10.8.1), suppose f is p times continuously differentiable (resp. E is jinite dimensional and f is indejnitely differentiable, resp. E is finite dimensional and f is analytic) in I x H. Then it is possible to choose J and V such that the function ( t , t o , x,) u(t, t o ,x,) is p times continuously differentiable (resp. indejnitely differentiable, resp. analytic) in -+

J X J X V .

+ s, t o , x,) - x,,

Indeed, if we write u(s, t o , x,) = u(t, s -+ u(s, t o , x,) is a solution of the equation

z' = f ( t ,

we see that

+ s, x, + z )

which takes the value 0 at the point s = 0; the result then follows from (10.7.4) and (10.7.5).

For linear differential equations, there are much more precise results. The equation

x'

(10.8.3)

= A(t)*

x

is called the homogeneous linear differential equation associated to (1 0.6.2) ; the difference of any two solutions of (10.6.2) in I is a solution of (10.8.3) in I, and the solutions of (10.8.3) in I constitute a vector subspace 2 of the space qE(1) of all continuous mappings of 1 into E.

(10.8.4) For each (s, x,), let t --+ u(t, s, x,) be the unique solution of (10.8.3) dejned in I and such that u(s, s, x,) = x, . (1) For each t E I, the mapping x, -+ u(t, s, x,) is a linear homeomorphism C(t, s) E 9 ( E ) of E onto itself. (2) The mapping t -+ C(t,s) of I into the Banach space 9 ( E ) is equal to the solution of the linear homogeneous differential equation (10.8.4.1)

U'

= A(t)0

U

which is equal to I, (identity nzapping of E) for t = s. (3) For any three points r, s, t in 1 (10.8.4.2)

C(r, t ) = C(r,s) C(s, t ) 0

and

+ +

C(s, t ) = (C(t,s))-'

It is clear that u(t, s, xl) u(t, s, x2) (resp. Au(t, s, x,)) is a solution of (10.8.3) which is equal to x1 xz (resp. Ax,) for t = s; hence (10.6.4) it is equal to u(t, s, x1 xz) (resp. u(r, s, Ax,)) in I, which proves that the mapping x, -+ u(t, s, x,) is linear; let us write it C(r,s) (we have not yet

+

proved that this mapping is continuous in E).

306

X

EXISTENCE THEOREMS

Now the bilinear mapping ( X , Y ) + X o Y of 2 ( E ) x 9 ( E ) into 2 ( E ) is continuously differentiable (8.1.4); denote by R ( t ) the continuous linear mapping U - i A ( t ) U of 2 ( E ) into itself. From (5.7.5) it follows at once that 0

IIWt)

- W)ll G

I I 4 t ) - 4411

hence, if K = R, t -+ R ( t ) is regulated if t + A ( ? ) is regulated. On the other hand, if t -+ A ( t ) is differentiable, so is t R ( t ) , and its derivative at the point t (identified (Section 8.4) to an element of 2 ( E ) ) is the mapping U-+ A ’ ( t ) U ((8.1.3) and (8.2.1)); hence if t -+ A ’ ( t ) is continuous, so is t -+ R’(t). We can therefore conclude that if K = C and if t + A ( r ) is analytic in I , so is t -+ R(r) (9.10.1). In any case, we may apply (10.6.4) to the equation (10.8.4.1); let V ( t )be the solution of that equation equal to IE for t = s. We have, for any t E I ((8.1.3) and (8.2.1)) 0

D(V(t) * x0) = V’(t) . x0 = A ( t ) . ( V ( t )* xO) and furthermore, for t = s, V ( s ). xo = ZE . xo = x,; it therefore follows from (10.6.4) applied to (10.8.3) that C(t,s) . x,, = V ( t ) . xo for any x,, E E, hence C(t,s) = V(r) for t E I . This proves that C(r,s) E 2 ( E ) and that t 3 C(t,s) is the solution of (10.8.4.1) which is equal to IEfor t = s. Finally, the function r --t C(t,v ) . xo is the solution of (10.8.3) equal to C(s, r ) * xo for t = s; hence, by definition

C(t, v ) . xo = C(t,s) . (C(s, v ) * xo) = (C(t,s) C(s, r ) ) * x, 0

for any xo E E, which proves the first relation (10.8.4.2); as C ( t , t ) = Z,, that relation yields C(t,s) 0 C(s, t ) = ZE. This shows that C(s, t ) is a hijectire linear mapping of E, whose inverse mapping is C(t,s) (hence also belongs to 2(E)). With this we reach the end of the proof of (1 0.8.4). The operator C(t,s) is called the resolvent of (10.8.3) (or of (10.6.2)) in I. (103.5) The niupping (s, t ) -+ C(s, t ) of 1 x I into 2 ( E ) is continuous. We may indeed write C(s, t ) = C(s, to) (C(r, to))-’, and the result then follows from (10.8.4), (5.7.5), and (8.3.2). 0

The knowledge of C(s, t ) enables one to give the explicit solution of (10.6.2) taking the value xo for t = t o : (1 0.8.6)

The function U(t) =

C(?, to) . xo

+

9 THE THEOREM OF FROBENIUS

307

is the solution of (10.6.2) in I which is eyuul to xofor t = to (if K = C, the integral is to be taken along the segment of origin r, and extremity t ) .

Indeed, one may write, by (10.8.4.2)

using (8.7.6); therefore, we have u ( t ) = C(t,to) . z ( t ) , where z ( t ) = xo

Hence ((8.1.4) and (8.2.1))

+

lo

( C ( t o ,s) . b(s))ds.

d ( t ) = C ’ ( t , to) * z ( t )

+ C(t,1,) - z’(t).

But by (10.8.4.1), C‘(t,to) = A ( / ) C(t, to), and on the other hand, z’(t) = C(to,t ) . b(t) by definition; hence 0

-

u’(t) = A ( / ) u ( t )

and as u(t,)

= xo , this

+ b(t)

ends our proof.

When E = K“, and the equation (10.6.2) is then written as a system of scalar linear differential equations (10.6.5), the resolvent C(s, t ) is an invertible n x n matrix (cij(s,t ) )whose elements are continuous in I x I, and t -+ c i j ( t , s) is a primitive of a regulated function in I if K = R, an analytic function in I if K = C.

PROBLEM

(a) Suppose, in the linear differential equation (10.6.2), that A and b are analytic functions in the simply connected open subset H c C. Show that, for any toE H and any x,, E E, there is a unique solution u of (10.6.2), defined in H and such that u(fo) = x o . (Use the same kind of argument as in (9.6.3): (10.6.3) allows one to define a solution of (10.6.2) along a broken line (Section 5.1, Problem 4) in H, and the argument of (9.6.3), along with local uniqueness, yields the result.) (b) Show that the result of (a) is not valid for the scalar differential equation x ’ = f / x : given any simply connected open subset H c C , and any fo E H, there exists an xo E E, such that xo # 0 and that there is 170 solution of the equation defined in H and equal to xo for t = lo,

9. T H E T H E O R E M OF F R O B E N I U S

Let E, F be two Banach spaces over K , A (resp. B) an open subset of E (resp. F), U a mapping of A x B into the Banach space 2 ( E ; F) (Section 5.7).

308

X

EXISTENCE THEOREMS

A differentiable mapping u of A into B is a solution of the total differential equation

(10.9.1 )

Y’ = U ( x ,Y )

if, for any x E A , we have

(10.9.2)

u’(x) = U(x,U(.Y)).

When E = K, 9 ( E ; F) is identified to F (5.7.6), and a total differential equation is thus an ordinary differential equation (10.4.1). When E = K“ is finite dimensional, a linear mapping U of E into F is defined by its value at each of the n basis vectors of E, and, by definition, (10.9.2) is thus equivalent to the system of ii “partial differential equations” (10.9.3)

D i y = f i ( x l ,. . . , x, , y )

(1

< i < n).

In general, such a system will have no solution when 11 > 1 , even if the right-hand sides ,f; are continuously differentiable functions. We say that an equation (10.9.1) is conipletely integrable in A x B if, for every point (xo,yo) E A x B, there is an open neighborhood S o f x o in A such that there is a unique solution u of (10.9.1), dejned in S, with values in B, and such that 4 x 0 ) = Yo. We will suppose in what follows that U is continuously chfferentiuble in A x B; for each (x,y ) E A x B, D, U ( x ,y ) (resp. D, U ( x ,y ) ) is an element of 9 ( E ; 9 ( E ; F)) (resp. 9 ( F ; 9 ( E : F))), which can be identified to the continuous bilinear mapping (sl, s,) -+ (D, U(x,y ) . sl) . s, of E x E into F, written (sl, s2) -+ D, U(x,y ) . (sl, s,) (resp. the continuous bilinear mapping ( t , s) -+ (D,U(x, y ) . t ) . s of F x E into F, written (1, s) -+ D, U ( x ,y ) . ( t , s)) (5.7.8); furthermore, the linear mapping s1 + ( D 1U ( x ,y ) . sl) s, of E into F, for each s, E E, is the derivative at the point (x, y ) of the mapping x + U(x,y ) . s2 of E into F, by (8.2.1) and (8.1.3); similarly, the linear mapping t -+ (D, U(x,y ) t ) * s of F into F, for each s E E, is the derivative at the point (x, y ) of the mapping y -+ U(x,y ) . s of F into F. 1

(10.9.4) (Frobenius’s theorem) Suppose U is continuously differentiable in A x B if K = R, t,tIice continuously riifferentiuble if K = C. In orrler that (10.9.1) be conipletely integrable in A x B, it is necessary and sufljcient that, for each (x,y ) E A x B, the relation (10.9.4.1) Di u(X,Y )

(Si, $2)

(u(x,J’) ’ S1, S2) + D, U(x, Y>. (U(X,Y ) . s 2 , $1)

-t D, U(x,Y ) *

= D, U.u,Y> . ( s 2

9

holds,for any pair (sir s,) in E x E.

s1)

9

THE THEOREM OF FROBENIUS

309

(a) Necessity. Suppose u is a solution of (10.9.1) in an open ball S c A of center xo such that zi(xo)= yo; then, from (10.9.2) and the assumption it follows that u‘(x) is differentiable in S ; moreover, for any s,EE, the derivative at the point xo of the mapping x -+ u’(x) . s, is sI -+ u”(xo) . (s,, s2)by (8.12.1). But by (10.9.2), that derivative is also (using (8.2.1), (8.1.3), and (8.9.1)) s1 -+

(D, U(x0 Y o ) * 3

Sl)

. s2

+ (D, U(x0 Yo) . (u’(x0)

*

7

s1))

*

s2.

Using the relation (10.9.2) again, and expressing that the second derivative of u at the point xo is a symmetric bilinear mapping (8.12.2), we obtain (10.9.4.1) at the point (xo, yo). But by assumption that point may be taken arbitrarily in A x B, hence the result. (b) Suficiency. Let So c A be an open ball of center xo a n d radius ct To c B an open ball of center yo and radius /3, such that U is bounded in So x To, let 11 U ( x ,y)II < M. We consider for a vector z E E the (ordinary) differential equation (where 5 E K) (10.9.4.2)

M” =

U(xo+ 52, w) * z

= f(5,

1.0,

z)

and observe that if u satisfies (10.9.2) in a neighborhood IIx - xoII < p of llzll < p is a solution of (10.9.4.2) in the ball < 1, in K , taking the value yo for 5 = 0 (which already proves uniqueness of u by (10.5.2)). Now the right-hand side of (10.9.4.2) is continuously differentiable for 151 < 2, IIM, - yo/l < /3 and llzll < a/2, and we have ,(fI[ M’, z)ll < Mllzll for such values. Applying (10.5.6) to f and to g = 0, we conclude that for any z E E such that ilzll < /3/2M, there is a unique solution 5 .+ v(5, z ) of (10.9.4.2) defined for 141 < 2, taking its values in H and such that a(0, z ) = y o . We are going to prove that thefimction u ( x ) = z*( I , x - xo) is a solution of (10.9.1) in the ball IIx - xoII < /3/2M. Now, for lizll < /3/2M and 151 < 2, we know from (10.7.3) that u is continuously differentiable, and that t -+ D, ~ ( 5z,) is, for 151 < 2, the solution of the linear differential equation

xo,5 -,u(xo + 5 2 ) for

v‘ = D, f ( 5 , u ( 5 , z), Z ) v + D, f(5, 4 5 , z), Z ) taking the value 0 for 5 = 0. For any sI E E, write g(5) = D, v(5, z) s,; we , z ) . g ( 5 ) + D, f(5, 4 5 , z), z ) . s1 and from the have g ’ ( 0 = D,,f(5, ~ ( 5z), definition 0f.L this can be written

d(5)= NO . M5L 4 + B(5). s1 + W(5) (s1,4 *

O 5z, ~ ( 5z>), , 4 5 ) = D2 U(xo + 52, 4 5 , z)), B(t) = ~ ( X + C(5) = D, U(xo+ 42, v(5, z)). We want to prove that g(5) = 5U(xo + 52, u(5, z)) * s1

with

310

X

EXISTENCE THEOREMS

and we therefore consider thedifferenceh(t)

-

g(5) - tB(5) sl.We have

= g ( ( ) - (U(x0

+ ~ z , v ( (z)), 's1=

h ' ( 0 = A(5) . Mt), z>f B(5) . s1 + W(5). (S1r

4 (K5) z , Sl)

- B(5) . s1 - t C ( 0 . ( z , $1 -
5 ) . (B(5) z , s1) *

= C(5)

*

(sly

4 + 4 5 ) . (B(5)

*

s l y

4

hence

h ' ( 0 = 4 5 ) .( g ( 0 - 5&5)

Sl,Z>

*

=4

5 ) (Iz(t),z). *

Furthermore, h(0) = 0; but the only solution of the linear differential equation r' = A ( < ) . (r, z ) which vanishes for ( = 0 is r(5) = 0 (10.6.3),hence h(5) = 0 for 151 < 2, which proves the relation

D,

4 4 , Z)

*

~1 =

5u(xo + 5z, 4T,z))

. ~1

for any s1 E E, i.e. D, v(5, z) =
+

(10.9.5) Suppose U is contitiuously differentiable in A x B i f K = R, twice continuously differentiable if K = C, and ueriJies the Frobenius condition (10.9.4.1).Then, for each point (a, b) E A x B, there is an open ball S c A of center a and an open ball T c B of center 6 , having the following properties: ( I ) for any point (xo,yo) E S x T, there is a utiique solution x -+ u(x, xo ,y o ) of (10.9.1),defined in S and such that u ( x o , xo,y o ) = y o ; ( 2 ) u is continuously differentiable in S x S x T. If in addition E aid F are jinite dimensional arid U is p times continuously differentiable (resp. itideJiriitely differentiable, resp. analytic) in A x B, then u is p times coiitinuously differentiable (resp. indejinitely differentiable, resp. analytic) in S x S x T. Finally, there is an open ball W c T of center b such that, f o r every point ( x , xo,yo) E S x S x W, the equation y o = u ( x o , x, y ) has a unique solution y = u(x, xo, y o ) in T. Let So c A be an open ball of center a and radius a, To c B an open ball of center b and radius 0, such that (1 U ( x ,y ) ( (< M in So x T o . Consider the ordinary differential equation

(1 0.9.5.1)

W' =

U(X,

+ 52, yo +

. z =f (<,

IV)

\\', Z, x O ,

yo).

9

THE THEOREM OF FROBENIUS

311

As in the proof of (10.9.4) we see that there is a unique solution t+u(t, z, xo ,yo) of that equation, defined for 151 < 2 and such that v(0,z, xo,yo) = 0, provided /Ixo- all < ~ / 8 , llzll < inf(cc/4, b/2M), /lye - blj < p. Furthermore, (10.7.3) shows that v is continuously differentiable for these values of (, z, xo, yo provided E and p have been taken such that the derivative of U is bounded in So x To. Then (10.9.4) shows that u(x, xo,yo) = y o + u(1, x - xo, xo, y o ) is the unique solution of (10.9.1), dejined i,? S: I/x- all d ~ / 8 , taking the value y o for x = xo, hence (x, xo, yo) + u ( x , xo , y o ) is continuously differentiable in S x S x T o . If E and F are finite dimensional, the proof that u is p times continuously differentiable (resp. indefinitely differentiable, resp. analytic) when U has the corresponding property, is done in the same way, using (10.7.4) (resp. (10.7.5)) instead of (10.7.3). Finally, the last statement of the theorem is proved by the same argument as part (c) of (10.8.1 ).

When E = K", the Frobenius condition (10.9.4.1) of complete integrability is equivalent, for the system (10.9.3), to the relations

a

=-fj(xl,

axi

...

a

(where it must be remembered that -fi(xl, . . . , x,, y ) is an element of aY 9 ( F ; F) (a matrix if F is finite dimensional), andfj(xl, . . . , x,, y ) an element of F).

CHAPTER XI

ELEMENTARY SPECTRAL THEORY

The choice of the subject matter of this chapter has been dictated by two considerations: ( I ) it is the first step in one of the main branches of modern functional analysis, the so-called “ spectral theory ; (2) it draws practically on e w r y preceding chapter for the formulation of its concepts and the proof of its theorems, and thus may convince the student that the “ abstract developments of these chapters were not purposeless generalizations. General spectral theory, being closely linked to the general theory of integration, falls outside the scope of this first volume, and the reader will not find any results of that theory in this chapter, with the exception of the proof of the existence of the spectrum (11.I .3) and a few elementary properties of the adjoint of an operator (Section 11.5). We have concentrated on the theory of compact linear operators, which can again be considered as ‘‘ slight ” perturbations of general operators, although in a sense quite different from the one which was prevalent in Chapter X ; here what is considered as “ negligible is what happens in finite dimensional subspaces, and the substance of the main theorem (11.3.3) on compact operators is that when we add such an operator to the identity, what we get is again a linear homeomorphism, provided it is restricted to a suitable subspace offinite codinzension. Compact self-adjoint operators in Hilbert space have a special interest, not only because it is possible to have much more precise information on their spectrum than for general compact operators (11.5.7), but also because their general theory immediately applies to Fredholm integral equations with hermitian kernel (Section 11.6), and in particular to the classical SturmLiouville problem, which we have chosen as a particularly beautiful illustration of the power of the methods of functional analysis (Section 11.7). For more information on spectral theory, and on its powerful applications, we strongly recommend Courant-Hilbert’s classic [ 101 for its delightful style and its wealth of information. General spectral theory will be developed ”

”

”

31 2

1 SPECTRUM OF A CONTINUOUS OPERATOR

313

in Chapter XV, and its most important applications in Chapters XXI (representations of compact groups), XXII (harmonic analysis), and XXlII (linear functional equations). 1. SPECTRUM O F A CONTINUOUS OPERATOR

Let E be a complex normed space; a linear mapping u of E into itself is often called an operator in E. The set Y(E; E) of continuous operators (which we will write simply 2’(E)) is a complex normed space (Section 5.7); it is also a noncommutative algebra over C , the “ product being the mapping (u, c) u i’, also written (u, c) U L ~ . The identity mapping of E is the unit element of 2’(E), written 1 , . The mappings (u, + u + z’ and (u, u) + u u are continuous in Y(E) x Y(E) (5.7.5). We say that a complex number C is a regular value for a continuous operator u if u - [ 1 has an inverse in 2 ( E ) (i.e. is a linear homeomorphism of E onto itself). The complex numbers [ which are not regular for u are called spectral values of u and the set of spectral values of u is called the spectrum sp(u) of u. If [ E C is such that the kernel of u - i. 1, is not reduced to 0, then c is a spectral value of u ; such spectral values are called eigenualues of u ; any vector x # 0 in the kernel of u - [ . 1 E , i.e. such that u(x) = [ x , is called an eigenvector of u corresponding to the eigenvalue i;these eigenvectors and 0 form a closed vector subspace of E, the kernel of u - [ . I,, also called the eigenspace of u corresponding to the eigenvalue [, and written E([) or E(i; u). When E has jinite dimension 1 7 , elementary linear algebra shows that any spectral value of an operator u is an eigenvalue of u (A.4.19); the spectrum of u is a finite set of at most I I elements, which are the roots of the characteristic p o l ~ ~ 0 / 7 1 det(u i a l - ( . IE)of u, of degree t? (A.6.9). But if E is infinite dimensional, there may exist spectral values which are not eigenvalues. ”

--f

--f

11)

Example (11.1.1) Let E be a complex Hilbert space, (an)n2la total orthonormal system in E (6.6.1). To each vector x = Ei,,a, in E (with llxll’ = ~ l C f l I 2 )we n

n

associate the vector u(x)= E[,,a,,+,; it is readily verified that u is linear and . IIu(x)/I = IlxII, hence (5.5.1) u is continuous. Moreover, u(E) is thesubspaceof E orthogonal to a,, hence u is not surjective, and this shows that c = 0 is a spectral value of u ; but u ( x ) = 0 implies x = 0, hence 0 is not an eigenvalue of u. 1’

314

ELEMENTARY SPECTRAL THEORY

XI

(1 1 .I .2) Suppose E is a coniplex Banach space, u a continuous operafor in E. The set R, of regular elements iE C ,for u is open in C and the mapping i ( u - i. l E ) - ' of R,, into 9 ( E ) is analytic. Suppose io E R,, and let co = ( u -io . l E ) - ' . For any 5 E C, we may write, in Lf'(E), --f

-

'

- (0

But,by(8.3.2.1),for

(i- l o )

'

li-

'

'E =

( u - (0

1,)(1 - (i- iO)'O)

'

<: IIu0II-', I , - (i- <,)z~,hasaninversein~(E),

equal to the sum of the absolute convergent series

m

1 (i- C0)"z$;

hence, for

ll=O

these values of i,u - i . 1, is invertible in LP(E), and its inverse, equal to

m

(i- Co)nzl~+l, the series being absolutely convergent for li - l o /< I I Z ~ ~ I I - ' ; which ends the

(1, - (i- ~ o ) u o ) - ' z ~ ocan , be written ( u - i f1 J '

=

n=O

proof.

(11.1.3) If' E is a complex Banach space, the spectrum ?f any continuous operator u it7 E is a notiempty compact subset of C contained in the ball l i l d IlUII. First observe that for [ # 0, u - i . 1 = - ( ( l E - ( - ' u ) , and therefore u - ( . I , is invertible in Y(E) for > IIuil, by (8.3.2.1). Furthermore, for

l i l '111411,

where the series is absolutely convergent, and

a s soon as l i l 3 211~11,we have therefore ll(u - i. l,)-' /I d llull-'. Now, if we had R, = C, ( u - ( . IE)-' would be a n entire function (9.9.6), bounded in C since it is bounded in the compact set Ill d 211ull and bounded in its complement; by Liouville's theorem (9.11 . I ) ( u - [ . 1 , ) - I would be a constant, hence also its inverse LI - i . I , which is absurd. The first part of the proof shows in addition that ( u - i* I,)-' exists and is analytic for l i l > /lull, therefore the spectrum of II, which is closed in C, is compact and contained in the ball

,

lil d I I ~ I I .

It is possible to give examples of operators for which the spectrum is an arbitrary compact subset of C (see Problem 3).

1

SPECTRUM OF A CONTINUOUS OPERATOR

315

PROBLEMS

1. Let E be a complex Banach space, u an element of P ( E ) , sp(u) its spectrum. (a) Show that if a complex number 5 is such that, for a n integer p > I , 15Ip> lluPll, then 5 is regular for u. (Use (11.1.3), and from the convergence of the series m

"=O

5-"pu"p.conclude that the series

x <-"urnis also convergent.) m

n=O

(b) Show that the number p(u) = inf I l ~ " l l ' ~ is " equal to the radius of the smallest n

2.

3.

4.

disk of center 0 containing sp(u), and furthermore that the sequence (l~u"~ll'")has a limit equal to ~ ( u )(Use . (a), Problem 1 of Section 9.1, and (9.9.4).) (For an example in which p(u) f llull, see Section 11.4, Problem 4.) Let u, v be two elements of Y(E), where E is a complex Banach space. Show that, with the notations of Problem I , the intersections of SP(NU)and sp(vu) with C - {0}are the same. (Observe that if A g are two elements of Y(E) such that l E -fg is invertible, ghfis the inverse of 1 - gf) and h = (1 -fig)-', then 1 Let E be a separable complex Hilbert space, (en)">]a Hilbert basis of E. Let S be an arbitrary infinite compact subset of C, and let (p,,) be a denumerable set of points of S , which is dense in S (3.10.9). Show that there is a unique element L I E 9 ( E ) such that u(e,) = pne, for every n > 1 ; prove that the spectrum of u is equal to S , whereas the eigenvalues of u are the p,, . If 5 E S, 5 is not equal to any of the pn, and vc = u - 5 . l E, show that v : ( E ) is dense in E but not equal to E(use (6.5.3) to prove the first statement). Show that the Tpectrum of the operator u defined in (11.1.1) is the disk 151 S 1 in C; I I has no eigenvalue. If v c = u - 5 . I,, show that for 151 < 1, v,(E) is not dense in E, but for 151 1, vc(E) is dense in E and distinct from E (cf. (6.5.3)). Let E be a complex Banach space, Eo a dense subspace of E. Show that for any element I I f 9 ( E 0 ) , the spectrum of u contains the spectrum of its unique continuous extension ri to E (5.5.4). Give an example in which these spectra are distinct and an example of an operator u E Y(EO)and of a spectral value 5 of u such that, if u 5 = u - 5 . l E , v c is a bijective mapping of Eo onto itself (in Problem 3, consider the subspace Eo of E consisting of the (finite) linear combinations of the vectors en). Note that this is impossible if Eo is a Bunuch space, for every continuous bijective linear mapping of Eo onto itself is then a homeomorphism (12.16.8). Let E be a complex separable Hilbert space, (en),,>] a Hilbert basis of E; let u be a continuous operator in E such that, for every pair of indices h, k , one has (&,)I e x )> 0. (a) Show that the number p(u) (Problem I(b)) belongs to sp(u). (Note that for 151 > &I), if v c (u - 5 . I E ) - I , one has

+

:

5.

6.

:

m

(O;(eh)

I 4 = -"C (un(eh)I e.d5-"-' =O

and use Problem 7(b) of Section 9.15.) (b) Suppose in addition that for some integer n > I , there exist an integer k > 1 such that (u"(ek)I e b )= d > 0. Prove then that p(u) > d''" (observe that for every integer m > I , (unm(ek) I e r )> d"). (c) Suppose p(u) > 0 and that the point p(u) is a pole of the function 5 + u c . Prove

x 5" m

that there exists then an eigenvector x =

5" > 0 for every n. (Let N

en of u corresponding to p(u), such that

" = I

be the order of the pole p(u) of v c , and let

316

XI ELEMENTARY SPECTRAL THEORY

Show that (wN(eh)I ek)< 0 for every pair h, k , and w N # 0 by assumption, and use the fact that U W N = p(u)wN .) (d) Suppose that (u(el) I eA)> 0 for every pair of indices h, k (this, by (b), implies p(u) > 0), and in addition suppose that p(u) is a pole of v c . Show that p ( u ) is then a simple pole of v z . (Observe that ( d / d ( )- v c = v t and prove that, if one had N > I , then one would have wA = 0; this would imply (wN(ek)I eA)= 0 for every k ; using the relatidn uwN = p ( u ) w N , observe that if ( w N ( e k ) 1 eh) = 0 for one index h , then wN(eA) 0.) Prove that there exists an eigenvector z = = x(.en of N corresponding to p ( u ) and such that 1

=c~ “ e is, an eigenvector of the adj,oint u* corresponding to p ( u ) (cf. Section one has 7. > 0 for every n, or 7“ 0 for every (otherwise, one would get the contradictory inequality In I 7. I 0 for every n. Next show that if y

n

=Z

11.5),

II

n

majoration of each 7” derived from p(u)T. =C qr(u(ek)1 en)).Conclude finally that all k

eigenvectors of u corresponding to p(u) are scalar multiples of z (exchange u and (“Theorem of Frobenius-Perron.”)

id*).

2. COMPACT OPERATORS

Let E, F be two normed (real or complex) spaces; we say that a linear mapping u of E into F is conTpact if, for any bounded subset B of E, u(B) is relatively compact in F. An equivalent condition is that for any bounded sequence (x,) in E, there is a subsequence (x,J such that the sequence ( ~ ( x , ~ ~ ) ) converges in F. As a relatively compact set is bounded in F (3.17.1), it follows from (5.5.1) that a compact mapping is continuous.

Examples (11.2.1) If E or F is finite dimensional, every continuous linear mapping of E into F is compact (by (5.5.1), (3.17.6), (3.20.16), and (3.17.9)).

(1 1.2.2) If E is an infinite dimensional normed space, the identity operator in E is not compact, by F. Riesz’s theorem (5.9.4). (11.2.3) Let I = [a, b] be a compact interval in R, E = gC(l) the Banach space ofcontinuouscomplex-valued functions in I (Section 7.2), (s, t ) -+ K(s, t ) a continuous complex-valued function in I x I . For any function f ’ E,~ the mapping r --t K(s, t ) f ( s )ds is continuous in I by (8.11.1); denote this

Jab

2 COMPACT OPERATORS

317

function by U f . Then the mapping f + Uf of E into itself is linear; we prove that it is compact. Indeed, if g = U j ; we can write, for to E I , t E I,

s(r>- d t o ) =

(1 1.2.3.1)

(K(s,

r ) - K(s, t o ) ) f ( s )d s .

.lab

As K is uniformly continuous in I x I (3.16.5),for any E > 0 there is a 6 > 0 such that the relation It - tol < 6 implies IK(s, t ) - K(s, t o ) [< E for any s E I ; hence, for a n y f in E

(11.2.3.2)

Ig(t) - g(t0)l

d E(b - a>llfll

by the mean-value theorem. This shows that the image U(B) of any bounded set B i n E is equicontinuozrs at every point to of I (Section 7.5); on the other hand, for any t E I, we have similarly Ig(t)l d kllf I/ if IK(s, t ) l d k / ( b - a ) in I x I . By Ascoli’s theorem (7.5.7),U(B) is relatively compact in E.

(11.2.4) With the same notations and assumptions o n K as in (11.2.3),let now F be the space of complex-valued regulated functions in I (Section 7.6), which is again a Banach space, when considered as a subspace of the space g,-(l);Uf’ is then defined as in (11.2.3)for any f E F, and the inequality (1 1.2.3.2)still holds. The argument in (11.2.3)then proves that U is a coinpact mapping of F into E.

(11.2.5) If’u,

L’

are two compact mappings of E into F, u + u is compact.

Let (x,) be a bounded sequence in E ; by assumption, there is a subsequence (x;) of (s,)such that (u(x;)) converges in F. As the sequence (x:) is bounded in E, there is a subsequence ( x i ) of (x;) such that (c(xi)) converges in F. Then by (3.13.10)and (5.1.5), the sequence (u(x1)+ xi)) converges in F. Q.E.D.

(1 1.2.6) Let E, F, El, F, be normed spaces, f’ a continuous linear mapping into E, g a continuous linear mapping of F into F,. Then,f o r any compact mapping u of E into F, u1 = g u o f is a compact niapping of El into F,. of El

For if B, is bounded in E 1 , f ( B l ) is bounded in E by (5.5.1), u ( f ( B , ) ) is relatively compact in F by assumption, and g(u(,f(B,))) is relatively compact in F, by (3.17.9).

318

XI

ELEMENTARY SPECTRAL THEORY

(11.2.7) I f u is a compact mapping of E into F, the restriction of u to any rector subspace El of E is a compact mapping of El into u(E,).

For by (11.2.6), that restriction is a compact mapping of El into F. If B is a bounded subset of El,-) is then a compact subset of F, and as __ - u(B) c u(E,), u(B) is relatively compact in u(E,). Example (11.2.8) With the same notations and assumptions on the function K as in (11.2.3), let now G be the prehilbert space defined by the scalar product

( f ) g ) = ~ a b f ( t ) g ( r ) don t the set gC(I)(6.5.1); we write the norm f)”’ = ~ ~ , to f ~ distinguish ~ a it from the norm l l f l i = supIf(t)l, and we still

(fI

f E l

with the norm llfll; the identity mapping f + f o f denote by E the space gC(l) E into G is continuous, since i l f i l z < ( b - a)’/’ * llfll by the mean value theorem; but it is not bicontinuous, nor is G a Banach space. The CauchySchwarz inequality (6.2.1) is written here

With the same notations as in (1 1.2.3), we therefore deduce from (11.2.3.1) and (11.2.8.1) that It, - t,l < 6 implies

Is(t1) - g(t2)I < E(b .llflla and similarly Ig(t)l < k(b . 1l.f 11’ for any t E 1. Hence, by the same

(1 1.2.8.2)

9

argument as in (11.2.3), f - Uf is a compact mapping of G into E; and as the identity mapping of E into G is continuous, ,f -+ U ’ is also a compact mapping of G into G by (11.2.6). (11.2.9.) Let E, F be two Banach spaces, E, (resp. F,) a dense subspace of E (resp. F), u a compact mapping of E, into F, , ii its unique continuous extension as a mapping of E into F (5.5.4). Then u“(E) c F, , and u” is a coiiipact mapping ofE into F, .

I t is immediate that any ball llxil < r in E is contained in the closure of any ball of center 0 and radius > Y i n E, (3.13.13) hence any bounded set in E is contained in the closure of a bounded set B in E, . But ii( f3) is contained in the closure in F of the set C(B) = u(B) by (3.11.4); now, u(B)

2 COMPACT OPERATORS

319

is relatively compact in F,, i.e. its closure in F, is compact, hence closed in F, and therefore equal to its closure in F. This shows that G(B) is contained in F, and relatively compact in that space. Q.E.D. (11.2.10) Let E be a normed space, F a Banach space, (u,) a sequence of nzuppings in Y ( E ; F) (Section 5.7) w~hichconverges to u in Y(E; F). Then, if every u, is compact, u is compact. Let B be any bounded set in E ; as F is complete, all we have to do is to prove that u(B) is precompact (3.17.5). Now B is contained in a ball /IxJId a ; for any E > 0, there is no such that n 2 no implies /Iu - u,II 6 6 / 2 4 and therefore (by (5.7.4)) IIu(x)- u,(x)II 6 ~ / for 2 any x E B. But as u,,(B) is precompact, it can be covered by finitely many balls of centers y j (1 6 j < m) and radius ~ / 2For . any x E B, there is therefore a j such that IIu,,(x) - yjll Q 4 2 , hence IIu(x) - yj/l 6 E , and the balls of centers y j and radius E cover u(B). Q.E.D. In particular, any limit in Y(E; F) of a sequence of mappings of finite rank is compact by (11.2.1) and (11.2.10). Whether conversely any compact mapping is equal to such a limit is still an open problem (see Problem 4).

PROBLEMS

Let E be a Banach space, A a bounded open subset of E, F a finite dimensional vector space. Show that for any p 3 1, the identity mapping f-f of the Banach space 9LP'(A) (Section 8.12, Problem 8) into 9Pp-l)(A)(the latter being replaced by %$' (A) for p I ) is a compact operator. (Use the mean value theorem and Ascoli's theorem.) Let u be a compact mapping of an infinite dimensional Banach space E into a normed space F. Show that there is in E a sequence (x,) such that IIxJ = 1 for every n, and lim u(x,) = 0. (Observe that there is a number a > 0 and a sequence (y") in E such that "+a,

l / y l l = 1 for every n, and lly,, - y, I/ > a for m # n (Section 5.9, Problem 3, and (3.16.1)), and consider the sequence (~(y,,)).) Conclude that if the image by u of the sphere S : /lx/l= 1 is closed in F, it contains 0.

Let E be a separable Hilbert space, (e,) a Hilbert basis of E. If u is a compact mapping of E into a normed space F, show that the sequence (u(e.)) tends to 0. (Use contradiction, and show that it is impossible that the sequence (u(e,)) should have a limit 0 # 0 in F.) If, conversely, F is a Banach space and the series of general term /lu(e,)lj2 is convergent, show that u is compact (use the Cauchy-Schwarz inequality to prove that the image of the ball l/xl/< 1 by u is precompact). Let F be a normed space having the following property: there exists a constant c > 0 such that, for anyfinite subset ( a i ) 1 6 i bof n F, and any E > 0, there exists a decomposition E = M N of E into a direct sum of two closed subspaces, such that M is finite dimensional, d(u,, M) < E for I < i < I ? , and if for any x E F, x = p ( x ) q ( x ) , where

+

+

320

XI

ELEMENTARY SPECTRAL THEORY

< c . d(x, M). Show that, under that assumption, any compact linear mapping of a normed space E into F is a limit in Y(E; F) of a sequence of linear mappings of finite rank (use the definition of precompact spaces). Show that any Hilbert space satisfies the preceding condition, as well as the spaces ( c d (Section 5.3, Problem 5 ) and I' (Section 5.7, Problem 1). 5. Let I = [ a ,b ] be a compact interval in R, K(s, t ) a complex valued function defined in I x 1, and satisfying the assumptions of Section 8.11, Problem 4. Show that if U is defined as in (11.2.3), U is still a compact mapping of E = %',(I) into itself. p ( x ) E M and q(x) E N, then llq(x)ll

3. T H E T H E O R Y OF F. RlESZ

We will need repeatedly the following lemma: (11.3.1) Let u be a continuous operator in a normed space E, v = 1, - 11, L, M two closed vector subspaces of E such that M c L, M # L, and v(L) c M. Then there is a point a E L n M such that llall < 1 and that, for any x E M, lMa>- u(x>Il 2 t .

c

By assumption, there is b E L such that b 4 M, hence d(b, M) = CY > 0. Let M be such that \lb - yll < 2a, and take a = ( b - y)/lib - y / l ; we have llall = 1, and, for any z E M, a - z = ( b - y - Ilb - yllz)/llb - yll; but as y + 116 - yllz E M , we have 116 - y - Ilb - yllzll 2 a, hence ]la - zil 2 for any z E M. But, for x E M, we have u(a) - u(x) = a - (x v(a) - ~ ( x ) )and , by assumption, x u(a) - v(x) E M ; hence our conclusion. y

E

+

+

+

(11.3.2) Let u be a compact operator in a normed space E, and let v = I, - u. Then: (a) the kernel v-'(O) isfinite dimensional; (b) the image v(E) is closed in E; (c) ti(E) hasjinite codimension in E; (d) if ~ ~ ' ( = 0 {) 0 } , then v is a linear honieomorphism of E onto v(E) (cf. (11.3.4)).

(a) For any x E N = v-'(O), we have u(x) = x , hence the image of the ball B: llxll < 1 in N by u is B itself; by assumption u(B) is relatively compact in E, hence in N since N is closed in E. But this implies that N is finite dimensional by Riesz's theorem (5.9.4). there is then a sequence (x,) in E, such that (b) Suppose y = lim ~(x,,) (3.13.13). Suppose first that the sequence (d(x,,, N)) is n-+ m

unbounded; then, by extracting a subsequence, we may suppose that

3 THE THEORY OF F. RlESZ

lirn d(x,, N)

=

+

00.

Let z,

= x,/d(x,

321

, N); it is immediate that d(z, , N) = 1,

fl+W

and therefore there is t, E N such that I/z, - t,l/ < 2. Let s, = z, - t,, and observe that by definition we have u(s,) = u(z,) = o(x,)/d(x,, N), and d(s,, N) = 1. From the assumptions we deduce at once that lirn u(s,) = 0. n+ w

But the sequence (s,) is bounded in E ; as u is compact, there is a subsequence (s,J such that (u(snk)) converges to a point a E E. As lirn (s, - u(s,)) = 0, we also have lirn s,, n-m

v(a) = lim u(s,,)

= a, = 0,

n+

w

hence, as x -+ d ( x , N ) is continuous, d(a, N)

=

1. But

and this contradicts the definition of N.

k-+ w

We therefore can suppose that the sequence (d(x,, N)) is bounded by a number M - I ; there is then a sequence x; such that x, - x; E N and Ilx;l\ 6 M ; as c ( x 3 = u(x,), we may suppose that IIx,II 6 M. Then as u is compact there is a subsequence (xnk) such that (u(xnk))converges to a point b E E ; as xnk- u(x,,) = ~(x,,) tends to y , (x,,,)tends to b + y and by continuity we have u(b + y ) = y , which proves that y E v(E), hence u(E) is closed. (c) T o say that u(E) has an infinite codimension in E means that there exists an infinite sequence (a,) of points of E such that a, does not belong to the subspace Vn-l generated by o(E) and by a,, . . . , a,-, for every n. Now each V, is closed since u(E) is closed (using (5.9.2)). By (11.3.1)we can define by induction a sequence (b,) such that 6, E V,, 6, $ V n - l , llb,/l 6 1, and IIu(b,) - u(bj)I/2 3 for any j 6 n - I . This implies that the sequence (u(b,)) has no cluster point, contradicting the assumption that u is compact. (d) In order to prove that 21 is a homeomorphism of E onto v(E) when V ' ( 0 ) = {0}, it is only necessary to show that for any closed set A c E, u(A) is closed in E (hence in o(E)) (3.11.4).But this is proved by exactly the same argument as in (b), replacing throughout E by A (and N by (0)).

(11.3.3) Under the same assumptions as in (11.3.2),define inductively N, = v - ' ( o ) , N , = U - l ( N k - l )for k > 1, F, = @), F, = V ( F k - 1 ) f o r k > 1. Then : (a) The N , form an increasing sequence of finite dimensional subspaces, the F, a decreasing sequence of finite codiniensional closed subspaces. (b) There is a smallest integer n such that N k + l = N , for k 2 n ; then Fk+l= F, for k Z n, E is the topological direct sum (Section 5.4)of F,and N,, and the restriction of 1: to F, is a linear homeomorphism of F, onto itself. (a) Define by induction

zl1

= 11, ti, = i I k - l

0

t'; I

claim that

ilk

= 1, - u,,

322

XI

ELEMENTARY SPECTRAL THEORY

induction on k , for U , and the result follows at once from the inductive hypothesis and from (11.2.6) and (11.2.5). Then by definition Nk = a ~ ' ( 0 and ) F, = zik(E), and our assertion follows from (11.3.2).

where Zlk =

(IE

uk

is

- Uk-1)

compact:

O

this

is

shown

by

( 1 -~ U ) = 1, - uk-1 - U -k uk-1

O

(b) Suppose Nk # Nk+1 for every k . We have U(Nk+,) C Nk for k 3 1 ; by (11.3.1), there would exist an infinite sequence (xk) of points of E such that xk E Nk, xk $ Nk-1, llxkll < 1 for k > 1 and IIu(xk) - U(Xj)II > f for any j < k . This implies that the sequence (u(xk))has no cluster point, contradicting the assumption that u is compact. Similarly, suppose Fk+l# Fk for every k . We have v(Fk) c F,+, for k > 1 ; by (11.3.1), there would exist an infinite sequence (x,) of points of E such that xk E Fk, xk $ Fk+l, llxkll < 1 for k 2 1, and IIu(xk) - U(Xj)II > for a n y j > k . This again implies contradiction, hence there exists a smallest integer m such that Fk+,= Fk for k 2 m . Next we prove that N, n F, = (0): if y E F, n N , , then there is x E E such that y = v,(x), and on the other hand v,(y) = 0; but this implies that tiZn(x)= 0, hence x E N,, = N, , and y = v,(x) = 0. By definition, we have F, c F, and v(F,) = F,; let us prove that F, = F,. Otherwise, we would have nz > n ; let z be such that z E F,-, c F,, and z $ F,; as U ( Z ) E F, = v(F,), there is a t E F, such that u(z) = v ( t ) , i.e. z - t E N, c N,; but as z - t E F,, we conclude that z = t , and our initial assumption has led to a contradiction. For each x E E we have v,(x) E F, = F,, and as u,(F,) = F, by definition of nz, there is y E F, such that v,(x) = u,(y), hence x - y E N, , and therefore E = F, + N , . This last sum is direct since F, n N, = ( 0 ) ; F, is closed and N, is finite dimensional, therefore (5.9.3) E is the topological direct sum of F, and N,. Finally, the restriction of I' to F, is surjective and its kernel is F, n N, c F, n N, = {0}, hence it is also injective. By (11.3.2(d)) that restriction is a linear homeomorphism of F, onto itself, and this ends the proof.

(11.3.4) Under the same assumptions as in (11.3.2), 11 is injectiile (i.e. v-'(O) = {0)), then v is surjective, hence a linear homeomorphism of E onto itself. For the assumptions imply that Nk = (0) for every k , hence n = 1 and N, is reduced to 0, therefore F, = E by (11.3.3) and the result follows from (11.3.3).

4

SPECTRUM OF A COMPACT OPERATOR

323

PROBLEMS

1. Let E, F be two Banach spaces, f a continuous linear mapping of E into F such that f(E) = F; then, there exists a number m > 0 such that for any y E F, there is an x E E for whichf(x) = y and Ilxll < mllyll (12.16.12). (a) If (y.) is a sequence of points of F which converges to a point b, show that there exists a subsequence ( y n k ) and , a sequence (x,) of points of E, which converges to a point a and is such that f(xk) = y,,, for every k . (Take (y,J such that the series of general term IIy.,+ - ynk/lis convergent.) (b) Let u be a compact mapping of E into F, and let u =f- u. Show that u(E) is closed in F and has finite codimension in F. (Follow the same pattern as in the proof of (11.3.2), using (a).) (c) Define inductively F1 = u(E), Fk+l = u(f-'(Fk)) for k > 1 ; show that there is an integer n such that = F, for k > n (same method). (d) Take E = F to be a separable Hilbert space, and let (en)nS1 be a Hilbert basis of E. Definefand u such that f(e,) = e n - 3for n > 4,f(e,) = 0 for n < 3, u(e,) = e.-z/n for n > 6, u(el)= u(e3)= 0, u(e2)= - e 2 , u(e4)= e l , u(e5) = e2 (e3/5). Define inductively NI = u-'(O), N,+,= u-'(f(Nk)) f o r k > I ; show that the N kare all distinct and finite dimensional. 2. Let E, F be two normed spaces,fa linear homeomorphism of E onto a closed subspace f(E) of F, u a compact mapping of E into F, and let u =f- u. (a) Show that u-'(O) is finite dimensional and u(E) is closed in F; furthermore, if u-'(O) = {O}, u is a linear homeomorphism of E onto u(E). (Follow the same method as in (11.3.2).) (b) Define inductively NI= u-I(O), N,+, = u-'(f(N,)) for k > 1 ; show that there is an integer n such that N k + = N,for k 2 n. (c) Give an example in which, when F1 = u(E), and F,,, = u(f-'(F,)) for k > 1 , the F, are all distinct (take for E = F a separable Hilbert space, and for f a n d u the adjoints (Section 11.5) of the mappings notedfand u in Problem l(d)). 3. Let E be a Banach space, g a continuous linear mapping of E onto itself such that Ilg11 <$; then f = I E -g is a linear homeomorphism of E onto itself (8.3.2.1). Let u be a compact operator in E, and let u =f- u ; then the statements in (11.3.2) and (11.3.3) are all valid. (First prove the following result, corresponding to (11.3.1): if M c L, M # L and u(L) c M, there is a n a E L n C M such that llajl < 1 and for any x E M such that IIxlI s I , M a ) - u(x)II > ( 1 - 211g11)/2.) 4. In the space E = I' (Section 5.7, Problem 1; we keep the notations of that problem), ) e 2 k f 2 (k > O ) , f ( e , )= eo, f(e2n+l)= let f be the automorphism of E such that f ( e Z k = e2k-1 for k I , and let u be the compact mapping such that u(e,) = 0 for n # 1 , and u(el)= e o . If u =f- u, and the Fkand N,are defined as in (11.3.3), show that Na+l # Nkand F, + # F, for every k .

+

4. SPECTRUM OF A C O M P A C T OPERATOR

(11.4.1 ) Let u be a compact operator in a complex normed space E. Then: (a) The spectrum S of u is an at most denumerable compact subset of C , each point of which, with the possible exception of 0, is isolated; 0 belongs to S if E i s injinite dimensional.

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(b) Each number A # 0 in the spectrum is an eigenvalue of u. (c) For each A # 0 in S, there is a unique decomposition of E into a topological direct sum of two subspaces F(A), N(A) (also written F(A; u), N(A; u)) such that: (i) F(A) is closed, N(A) i s j n i t e dimensional; (ii) u(F(2)) c F(A), and the restriction of u - A . 1, to F(A) is a linear homeomorphism of that space onto itself; (iii) u(N(A)) c N(A) and there is a smallest integer k = k(A), called the order oj'A (also written k(A; u)),such rhat the restriction to N(A) of(u - A . I , ) k is 0. (d) The eigenspace E(A) of u corresponding to the eigenvalue A # 0 is contained in N(A) (hence finite dimensional). (e) I f A, p are two difeerent points of S , distinct from 0 , then N(p) c F(A). (f) I f E is a Banach space, the function (-+ ( u - ( I,)-', which is dejned and analytic in C - S , has a pole of order k(A) at each point A # 0 of S. Let A # 0 be any complex number; as A-'u is compact, we can apply the Riesz theory (Section 11.3). By (11.3.4), if A is not an eigenvalue of u, I - A- ' u is a linear homeomorphism of E onto itself, and the same is true of course of u - A . 1, = -A(l, - A - l u ) , i.e. A is regular for u, which proves b). Suppose on the contrary A is an eigenvalue of u ; then the existence of the decomposition F(A) + N(A) of E with properties (i), (ii), (iii), follows from (11.3.3), as well as (d) (E(A) is the kernel noted N, in (11.3.3)). To end the proof of (c), we need only show the uniqueness of F(A) and N(A). Suppose there is a second decomposition E = F' N' having the same properties, and write v = u - A . 1., Then, any x E N' can be written x = y + z where y E F(A), z E N(A); by assumption there is h > 0 such that vh(x)= 0, hence vh(y) = 0; as the restriction t o F(A) of vh is a homeomorphism by assumption, y = 0 and x E N(1). This proves that N' c N(A), and a similar argument proves N(1) c N'. Next, if x = y + z E F' with y E F(n), z E N(A), we have vk(x) = vk(y), hence vk(F')c F(A); but as v(F') = F', this implies F' c F(A), Denote by u l , u2 the restrictions of u t o F(A) and N(A), respectively. From the relation (u2 - A . l N ( n ) ) k = 0, it follows by linear algebra (A.6.10 and A.6.12) that there is a basis of N(1) such that the matrix of u2 - I . IN(,, with respect to that basis is triangular with diagonal 0; if d = dim (N(A)), the determinant of u2 - ( lN(l)is therefore equal to ( A - ( ) d and this proves that u2 - ( . lN(n) is invertible if 5 # A. Let us prove on the other hand that u, - ( . lF(A)is invertible for ( - A small enough: we can write u1 - [ . 1 F(i) = v1 + (A - 5) . IF(,, with u1 = u1 - A . We know by (c) that V , is invertible; by (5.7.4), we therefore have I ~ V ; ~ ( X ) I I < IIv;' II * /Ixll in F(A), which can also be written IIu,(x)I/ 2 c . l/xil with c = llv;'/l-l. Now if i # 0 and

+

4 SPECTRUM OF A COMPACT OPERATOR

325

u1 - ( . IF(,) is not invertible, this implies, by (b) (applied to F(A) and u l , using (11.27)) that there would exist an x ;f; 0 in F ( I ) such that u l ( x ) = ('x, hence li- A1 . llxll = ilol(x)j/ 3 c . IIxll, which is impossible if - A1 < c. This shows that for [ # 0, i# I , and - I )< c, u - i .I, is invertible (since its restrictions to F(A) and N(A) are), i.e. ( is not in S; therefore all points A # 0 in S are isolated, and S is at most denumerable. By (b), for each A # 0 in S, there is x # 0 in E such that u(x) = Ax, hence 121 . ~~x~~ < 1 ~ . ~~x~~ ~ ~ by1 (5.7.4), and 111 < IIull, which proves S is compact. To end the proof of (a), suppose E is infinite dimensional; if u were a homeomorphism of E onto itself, the image u(B) of the ball B: ~1~~~ < 1 would be a neighborhood of 0 in E, and as it is relatively compact in E, this violates Riesz's theorem (5.9.4). If p is a point of S distinct from 0 and A, and x E N(p), we can write x =y z with y E F ( I ) , z E N(I). We have seen above that the restriction of w = u - /i . I, to N(A) is a homeomorphism; as wh(x)= 0 for h large enough, and wh(y)E F(I), wh(z)E N(A), we must have wh(y)= wh(z) = 0, which proves statement (e). in C - S follows If E is a Banach space, the analyticity of ( u - i. I,)-' from (11.1.2). With the same notations as above, A is not in the spectrum of u l , hence (by (11.2.7)) (ul - . IF(,,)-' is analytic in a neighborhood of A ; in particular, there are numbers p > 0 and M > 0 such that

+

for x E F(A) and l i - AI < p. On the other hand, we can write u2 -5 . I,(,, = (A - i). I,,,)+ 1i2 with u2 = u2 - A . I,(,, , and we know that for 5 # A, u2 -( . I,(,, is invertible; moreover, we can write

since 11; = 0. From this it follows that there is a number M' > 0 such that l i - 21k . l l ( u ~- i . IN(~.J-'(x)Il< M'llxll for l i - A1 < p, I # A and for any x E N(1). Now any x E E can be written x = y z with y E F(A), z E N(A), and there is a constant a > 0 such that llyll < a/lxl/and llzll < allxll (5.9.3); therefore we see that, for li - I1 < p, i# A, and any ~ E E we , have

+

l i - I l k ll(u - i. 1E)-'(X)Il

< 4 M p k + M')IIxII. < a ( M p k + M') for if A

In other words, l i - Alk . ll(u - i .l,)-'Il and - A1 6 p ; by (9.1.5.2), this implies that A is a pole of order < k for ( u - i* I,)-'. But by definition there is an x E N ( I ) such that z~:-'(x) # 0, hence (i-A)"-'((u - i I,)-'(X)) is not bounded when i# I tends to A, and this proves that A is a pole of order k , and ends the proof of (1 1.4.1). 9

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We say that the dimension of N(1) is the algebraic multiplicity of the eigenvalue A. of u, the dimension of the eigenspace E(1) its geometric n d tiplicity; they are equal if and only if k(1) = 1 ; when E is a Banach space, this is equivalent to saying that 1 is a simpre pole of ( u - 5 . l&'. (11.4.2) Let E be a Banach space, E, a dense subspace of E, u a compact operator in E,, ii its unique continuous extension to E. Then /he spectra of u and ii are the same, and for each eigenoalue A # 0 of u, N(A, u) = N(1, u"), E(1, u) =: E(1, ii) and k(1, u) = k(1, ii).

We know that u" is compact and maps E into E, , by (11.2.9); if A # 0 is an eigenvalue of u", any eigenvector x corresponding to 1 is such that x = 1-'C(x) E E,, hence A is an eigenvalue of u, and E(1, u") c E(1, u ) ; the converse being obvious, we have sp(ii) =sp(u) and E(A, u) = E(A, u") c E, for each eigenvalue A # 0. Considering similarly the kernels of ( u - A . IEJk and of its extension (ii - A . I E)k we see that they are equal, hence k(1, u) = k(A, u") and N(A, u) = N(1, u") c E, .

PROBLEMS

1. Let E be a coniplex Banacli space, u a compact operator in E; we keep the notations of (11.4.1), and in addition, we write pi (or p i , J and qA=- l E - p i the projections of E onto N(h) and F(h) in the decomposition of E as direct sum F(h) i N(h). (a) Show that -pi is the residue of the meromorphic function (u - 5 . I E ) - l at the pole h, for every A E sp(u) such that h .?; 0. (b) If h1, . .. , h, are distinct points of the spectrum sp(u), show that the projections p i , ( I < j < r ) commute, and that p i , I ' . . -+-pi, is the projection of E onto N(h,) -I . . . -t N(hJ in the decomposition of E as direct sum of that subspace and of F(hl) n F&) n . . . n F(h,). 2. Let E be an infinite dimensional complex Banach space, u a compact operator in E, (uJngl a sequence of compact operators in E, which converges to I I in the Banach space Y(E). (a) Prove that for any bounded subset B of E, the union u.(B) is relatively compact

u"

in E. (Show that it is precompact.) (b) If A E C does not belong to sp(u), show that there is an open disk D of center h and an integer no such that, for n > n o , the intersection sp(u,,) n D = 0(use (8.3.2.1)) and ( [ i n- 5 . IE)-' converges uniformly to (u - 5 . I E ) - I for 5 E D. (c) Let (p.) be a sequence of complex numbers such that p.~sp(u,) for every n ; such a sequence is always bounded. If h is a cluster point of ( p J , show that h E SP(N). (One can assume that h = lim pn 0; there is then x. E E such that llxn/l= 1 and u.(xn) = h.x,; use then (a).)

"'m

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SPECTRUM OF A COMPACT OPERATOR

327

(d) Conversely, let h # 0 be in sp(u). Show that for each n there is (at least) a number E sp(u,) such that h = lim p.. (Otherwise, one can assume that there is an open

p.

"+ m

disk D of center h and radius Y, such that D n sp(u) = {A} and D n sp(u.) = @ (extract from (u,) a suitable subsequence). Let then ^J be the road t + A Yeirdefined in [0,277];consider the integral

+

sy(lln 5 -

* IE)-l(5

-

d[

=0

for k

>0

and use (b) to obtain a contradiction.) (e) Let h # 0 be in sp(u), and let D be an open disk of center h and radius r such that D n sp(u) = {A}; there exists no such that, for n > no the intersection of sp(un) and of the circle 15 hl = r is empty (use (c)). Let pI,... , p, be the points of D n sp(u,),

c k(pj;

~

and write k , =

uJ. Show that there exists nl such that, for n

j=l

> n l , k, > k(h;u).

(Use the same method as in (d), multiplying (u, - 5 . by a suitable polynomial in 5 of degree k , .) Give an example in which k, > k(h; u) for every n. (f) With the notations of (e), let p =pi.", pn=

c pPJ,""; show that

j =

1

limp, = p

n- m

in the Banach space 9 ( E ) (use (b), and Problem I). Deduce from that result that there exists n2 such that, for n 2 n 2 , N, = N(p, ; u,) . . . N(p, ; u.) is a supplement to F(h; u) in E. (Suppose n is such that IIp --p.li < 1/2; if there was a point x, E F(h) n N. such that /lx, /I = I , then the relations p(x.) = 0, p,,(x,) = x, would contradict the preceding inequality. Prove similarly that the intersection of N(h; u ) and of the subspace F(pI ; u,) n . . . n F(p,;u,) is reduced to 0.) 3. Let u be a compact operator in an infinite dimensional complex Banach space E, and let P(5) be a polynomial without constant term; put v = P(u). Show that the spectrum sp(u) is identical to the set of numbers P(h), where h E sp(u); furthermore, for every p E sp(v), N(p; u ) is the (direct) sum of the subspaces N(h,; u ) such that P(&) = p, and F(p; u ) the intersection of the corresponding subspaces F(&; u). (Let V be any closed subspace of E, such that u(V) C V, and let uv be the restriction of u to V. Show that there is a constant M independent of V and n, such that ll(P(uv))'ll < M" lIuqI1. Apply that remark and Problem 1 of Section 11 .I, taking for V a suitable intersection of a finite number of subspaces of the form F(h; u).) 4. Let E be a separable Hilbert space, (en)n3oa Hilbert basis of E. Show that the operator u defined by u(e,) = e.+l/(n 1 ) for n > 0 is compact and that sp(u) is reduced to 0 (more precisely, u has no eigenualue). 5. Let u be a continuous operator in a complex Banach space E. A Riesz point for u is a point h in the spectrum sp(u) such that: (I) h is isolated in sp(u); (2) E is the direct sum of a closed subspace F(h) and of a finite dimensional subspace N ( h ) such that u(F(h)) c F(h), u(N(h))c N(h), the restriction of u - h . 1, to F(h) is a linear homeomorphism and the restriction of u - h . 1, to N ( h ) is nilpotent. (a) If h and p are two distinct Riesz points in sp(u), show that N(p) C F(h), and F(h) is the direct sum of N(p) and F(h) n F(p). (b) A Riesz operator u is defined as a continuous operator such that all points f 0 in the spectrum sp(u) are Riesz points. For any E > 0, the set of points h E sp(u) such that 1x1 > E is then a finite set { p i ,. . ., p , } ; let pz be the projection of E onto N(pJ in the decomposition of E into the direct sum N(pJ -t F(pJ ( I < i < Y), and let

+ +

+

u =u -

2u

i=l

0

p i . Show that sp(v) is contained in the disk

Problem I ) that lim I1u"Il"" n-m

< E.

151 < E , hence (Section 11 .I,

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(c) In the Banach space Y(E), let X be the closed (11.2.10) subspace of all compact operators. Show that, in order that u E Y(E) be a Riesz operator, it is necessary and sufficient that lim (d(u", Y ) ) ' '= " 0. (To prove that the condition is necessary, use n-rm

(b), observing that u" = u" i- w,, where w, is an operator offinirerank, hence compact. To prove that the condition is sufficient, use the result of Problem 3 of Section 11.3, which can be interpreted in the following way: if ilgIl< 4, then either h = 1 does not belong to sp(y u ) or is a Riesz point for y -t u.)

+

5. C O M P A C T OPERATORS IN H I L B E R T SPACES

Let E be a prehilbert space, u an operator in E. We say u has an adjoint if there exists an operator u* in E such that (11.5.1) for any pair of points x,y in E. It is immediate that the adjoint u* is unique (when it exists), and (by Section 6.l(V)) that then (u*)* exists and is equal to u. It is similarly verified that when the operators u and v have adjoints, then u + v, AM, and uv have adjoints respectively equal to u* + v*, Xu*, and u*u*. (11.5.2) If u is continuous and has an adjoint, then u* is continuous and lJu*JJ = JIuJ)in Y(E). Zf E is a Hilbert space, every continuous operator in E has an adjoint. From (11.5.1) and the Cauchy-Schwarz inequality (6.2.4) we deduce for any pair x, y ; taking x = u*(y), we get Ilu*(y)II d llull . llyll for any y E E, which proves the continuity of u* and the inequality JIu*JJ d JJuJ1; the converse inequality is proved by interchanging u and u* in the argument. If E is a Hilbert space and u is continuous, then, for any Y E E , the linear form x-+ (u(x)Iy) is continuous, and by (6.3.2) there exists a unique vector u*(y) such that (11.5.1) holds. From the uniqueness of u*(y), we conclude that u* is linear, hence the adjoint of u. The second statemento f (11.5.2) does not extend to prehilbert spaces. An operator u in a prehilbert space E is called self-adjoint (or hermitian) if it has an adjoint and if u* = u ; the mapping (x, y ) -+(u(x) I y ) = ( u ( y ) I x ) is

then a hermitian form on E; the self-adjoint operator u is called positioe (resp. nondegenerate) if the corresponding hermitian form is positive (resp. nondegenerate); one writes then u > 0. For any operator u having an adjoint, u + u* and i(u - u*) are self-adjoint operators.

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COMPACT OPERATORS IN HILBERT SPACES

329

(1 1.5.3) (i) If a continuous operator u in a prehilbert space E has an adjoint, = then u*u and uu* are selfadjoint posiiive operators, and j(u*uI( = (Juu*(( llu\lz = 11u*11’. In particular, i f u is self-adjoint, I I U ’ I / = 1 1 ~ 1 1 ~ . (ii) If P is the orthogonal projection of E on a complete vector subspace F (Section 6.3), P is apositive hermitian operator. Conversely, i f E is a Hilbert space, every continuous operator P in E which is hermitian and idempotent (i.e. P2 = P) is the orthogonal projection of E on the closed subspace P(E) (these operators are called the orthogonal projectors in LZ(E)).

(i) The fact that u*u and uu* are self-adjoint follows from the relations (u*)* = u and (uv)* = v*u*; moreover (u*u(x) I x) = (u(x) I u(x)) 2 0 for any x E E, and it is proved similarly that uu* is positive. Further this last relation shows that IIu(x)11’ < IIu*u(x)II . llxll by Cauchy-Schwarz, hence (by (5.7.4)) llul12 < IIu*uII. On the other hand, /Iu*uII < IIu*Il . llull = l\ull’ by (5.7.5) and (1 1.5.2), and this concludes the proof of (i). (ii) If P is the orthogonal projection of E on a complete subspace F, then (P . x l y - P y ) = 0 for x E E, y E E, hence (P . x Iy) = ( P . x IP - y ) = (x I P .y ) , which proves that P is hermitian, and it is positive since ( P x I x) = ( P x I P x) 2 0.Conversely, suppose E is a Hilbert space, and P2 = P = P*; as the then, for all x,y in E, ( P . x I y - P . y ) = ( x I P . y - P ’ . y ) = O ; relation y = P x implies P * y = P2 . x = P * x = y , P(E) is the kernel of 1, - P, hence is a closed vector subspace; furthermore, for any y E E, y - P y is orthogonal to every P . x,in other words to P(E), which proves (ii). (1 1.5.4) If E is a Hilbert space, the adjoint of any compact operator u in E is a compact operator. As E is complete, it will be enough to prove that the image u*(B) of the -

ball B: llyll < 1 isprecompact. Let F = u(B), which is a compact subspace of E, and consider, in the space %,-(F) (Section 7.2) the set H of the restrictions to F of the linear continuous mappings x -+ (x 1 y ) of E into C,where y E B; we prove that H is relatively compact in V,(F). Indeed, we have I(x - x’I v ) ~ < IIx - x’/I by the Cauchy-Schwarz inequality, since llyll < !, which shows that H is equicontinuous; on the other hand F is contained in the ball llxll < IIuII, hence I(x Iy)l < llull for anyy E B and a n y x E F ; Ascoli’s theorem (7.5.7) then proves our contention. Therefore, for any E > 0, there exist a finite number of points y j (1 < j < m) in B such that for any y E B, there is an indexj such that I(u(x) I y - yj)l < E for any x E B. But by (1 1.5.1 ) this last inequality is written I(x I u*(y) - u*(yj))I < E , and either u*(y) = u*(yj) or we can take x = z/llzll, where z = u*(y) - u*(yj);we therefore conclude that I/ u*(y) - u*(yj)II < E , and this ends the proof.

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Note that the proof that u*(B) is precompact still holds when E is not complete; but it can happen that in a prehilbert space E, a compact operator has an adjoint which is not compact.

(11.5.5) Let u be a compact operator in a complex preliilbert space E , having an adjoint u* which is compact. Then : (a) The spectrum sp(u*) is the image of sp(u) by the mapping 5 + [. (b) For each A # 0 in sp(u), k(A; u) = k(X; u*). (c) I f u = u - 1 * 1 , , then u*(E) is the orthogonal supplement (Section 6.3) of v-'(O) = E(A; u), and the dimensions of the eigenspaces E(A; u) and E(X; u*) are equal. (d) The subspace F(X; u*) is the orthogonal supplement of N(A; u), and the dimensions of N(A; u) and N(X; u*) are equal.

We have u* = u* - X l,, hence (u(x)I y ) = (x I u*(y)) from (11.5.1), and therefore the relation u(x) = 0 implies that x is orthogonal to the subspace v*(E). Now by (11.4.1) applied to u*, u*(E) is the topological direct sum of F(X; u*) and of the subspace u*(N(X; u*)) of N(X; u*), and from linear algebra (A.4.17) it follows that the codimension of u*(E) is equal to the dimension of u*-'(O) = E(X; u*); hence we have dim E(A; u) < dim E(X; u*). But u = (u*)*, hence we have dim E(A; u) = dim E(X; u*); furthermore, the orthogonal supplement of E(A; u) contains u*(E) and has the same codimension as v*(E), hence both are equal, which proves (c). This also shows that for any eigenvalue A # 0 of u, X is an eigenvalue of u*, and as the converse follows from the relation u = (u*)*, we have also proved (a). The same argument may be applied to the successive iterates vh of u, and shows that the image of E by u * ~= (oh)* is the orthogonal supplement of the kernel of vh. Using (11.3.2), (11.4.1), and the relation u = (u*)*, this immediately proves (b) and (d). Theorems (1 1.4.1) and (11.5.5) can be translated into a criterion for the solutions of the equation u ( x ) - Ax = y : (1 1.5.6)

Under tlze assumptions of (11.5.5) :

(a) If2 is riot in the spectrunz of u, the equation u(x) - Ax = y has a unique solution in E for every y E E. (b) IfA # 0 is in the spectrum of u, a necessary and suficient condition for y E E to be such that the equation u(x) - Ax = y have a solution in E is that y be orthogonal to the solutions of the equation u*(x) - Ax = 0.

5 COMPACT OPERATORS IN HILBERT SPACES

331

For a finite dimensional space, this reduces to the classical criterion for existence of a solution of a system of scalar linear equations.

(11.5.7) Let u be a compact self-adjoint operator in a complex Hilbert space E. Then: ( a ) Every element of the spectrum sp(u) is real and k(A) = 1 for every eigenvalue A # 0 of u. (b) If A, p are two distinct eigenvalues of u, the eigenspaces E(A) and E(p) are orthogonal. (c) Let (p,) be the strictly decreasing (finite or infinite) sequence of eigenvalues > 0 , (v,) the strictly increasing (finite or infinite) sequence of eigenvalues <0.For each k such that p k (resp. vk) is defined, let F; (resp. Fi) be the orthogonal supplement of E(pl) ... E(pk-l) (resp. E(v,) E(vk-l)); then p k (resp. vk) is the largest (resp. smallest) value of the function x -+ (u(x) I x ) on the sphere llxll = 1 in F; (resp. Fi),and thepoints of that sphere where (u(x) I x) = p k (resp. (u(x) I x ) = vk) are thepoints which belong lo E(pk) (resp. E(v)k). Furtherniore, llull = sup(pl, -vd. (d) The space E is the Hilbert sum (Section 6.4) of the subspaces E(pn), E(v,), and E(0) = u-'(O).

+

+

+ +

(It may happen that either the p, or the v, are absent, but from (c) it follows that the only case in which there are no eigenvalues # 0 is the case u = 0.) For any eigenvalue A # 0 of u, we have, for an eigenvector x corresponding to A, (u(x)I x ) = A(x I x), but (u(x)I x ) = ( x I u(x)) = (u(x)I x) is real for any x E E, hence, as ( x I x ) is real and # 0,1is real. If v = u - A . I,, we therefore have v* = v , hence v(E) is the orthogonal supplement of E(A) = v-'(O) by (11.5.5); this implies that the restriction of u to v(E) is injective, hence, by definition (see (11.3.3)) N(A) = E ( I ) , F(I) = v(E) and therefore k ( I ) = 1. This proves statement (a), and as E(p) = N(p) c F(A) for any eigenvalue p # I by (11.4.1 ), we also have proved (b). We first prove the last part of statement (c). Let p = sup(p,, - vl). Then by > p , whence it (11.1.2) the mapping [ -+ (u - [ lE)-' is analytic for follows at once that the mapping 5 + ( I E - tu)-' is analytic for < l/p. Now, for 5 in a sufficiently small neighborhood of 0, the power series m

n=O

runconverges to ( I E - @)-I

in 9 ( E ) (8.3.2.1); by (9.9.4) that power

< l/p. Furthermore, for each r such series converges for every { such that that 0 < r < l/p, if M is the maximum of Ij(lE - tu)-' 11 for 151 = r, the Cauchy inequalities (9.9.5) yield IIu"lI < M/r" < Mp". In particular, if we use (11.5.3),

332

XI

ELEMENTARY SPECTRAL T H E O R Y

we get here 1 1 ~ 1 1 ~ <" Mp2"for every n 3 1 ;taking 2"th roots and letting n tend t o + 00, we get, by Section 4.3, JIu/I< p. On the other hand we have p < llull by (11.1.3), hence llull = p. Let us now write (p,) for the strictly decreasing sequence of the absolute values of the eigenvalues of u, so that p1 = p = sup@,, -vl); and let G, be equal to the sum of the E(A) such that 111 = p, (there are of course either only one or two such eigenvalues A). Next let F, be the orthogonal supplement of G, + * * . + G,,-l; we have u(F,) c F, by (a), and we prove that the restriction u, of u to F, is such that llunll < p n - , . Otherwise, by what has just been seen above (and by (11.2.7)) there would be in F, an eigenvector x such that u(x) = Ax with 121 2 which contradicts the definition of F,. Write now x = y + z for every x E F,, with y E F,+l and z E G,; we have, by Cauchy-Schwarz,

- ll~,+lll . llYl12 + (44 I 4 < (u(x) 1x1 < l l ~ " + l I l* /lyl12+ (u(z) I z ) . Suppose p, = &, = -vk, and write therefore z = z1 + z2 with z , E E(p,,) and z2 E E(vk); this yields (u(z) lz) = pn(llz1ll2- I / Z ~ ~ / ~ ) . As / I X ~ / = ~ Ilyll2+ 11z1Il2 llz21I2, we see at once, using the preceding inequality and the inequality I I U , + ~ 11 < ,on,that on the sphere Ilxll = 1 in F,, the largest value of (u(x)I x) is p, and is reached at the points of E(ph) only, and the smallest value is - p n , and is reached a t the points of E(vk)only. The results are similar and simpler if either there is no k such that p, = - v k , or no h such that p, = p,,. Finally, if we remark that F;, = F,, E(v,) + . . . + E(v,) if ph = p, and s is the largest value of k such that p, < - vk , and similarly

+

+

Fi = F,

+ E(p1) + + E(p,.) * * *

if v k = -p, and r is the largest value of h such that p, < ph, an almost identical argument ends the proof of (c). Let now F, be the closed subspace, intersection of all the F,; by definition, u(F,) c F,, and there can be 110 eigenvalue # 0 of the restriction of u to F,; by (c) this implies that u(x) = 0 in F, . Furthermore, if a vector x E E is orthogonal to F, and to all the E(pk) and E(vk),by definition it is orthogonal t o all the G,, hence belongs t o F, , and being orthogonal to F, , it is 0. This proves (by (6.3.1)) that the algebraic sum of the subspaces E(pk), E ( v k ) , and F, is dense in E; hence, by (6.4.2), E is the Hilbert sum of those spaces.Any x E E can therefore be written uniquely x = xi + C xi + xo ,where x; ,xi, k

k

and xo are the Orthogonal projections of x on E(pk), E(vk), and F, , respectively, the sums being convergent series in E when the sets of indices are infinite (canonical decomposition of x); we conclude that

5

COMPACT OPERATORS IN HILBERT SPACES

333

and by the uniqueness of that expression, we see that u ( x ) = 0 implies x E F,; in other words, F, = u-'(O), which ends the proof of (11.5.7). Remarks (I1 -5.8) Let E, be a prehilbert space which is a dense subspace of a Hilbert space E (it can be proved that for any prehilbert space E, there is a Hilbert space E having that property; we have proved in (6.6.2) the special case of that theorem in which E, is separable). Let u be a compact self-adjoint operator in E,; then the results (a), (b), and (c) of theorem (11.5.7) hold without change for u. For it follows from the principle of extension of identities that the unique continuous extension u" of u to E is self-adjoint, and it is readily verified that llu"ll = llulj ; our assertion then follows from (11.4.2) and from the following remark: if F, is a finite dimensional subspace of E, , Go its orthogonal supplement in E,, G its orthogonal supplement in E, then Go is dense in G; this is a consequence of the fact that if u = I, - P,, in Y(E) (notations of Section 6.3), u is continuous and v(E) = G, u(E,) = Go (see (3.11.3)). With respect to the part (d) of (11.5.7), it is clear that the kernel of u is the intersection of E, with the kernel of 6, hence is the subspace of vectors of E, orthogonal to all eigenspaces E ( I ) with I # 0. But if we consider the canonical decomposition x = x i + x; + x , of an element x E E, , the

1k

1 k

sums on the right-hand side and the element x, do not necessarily belong to E , (11.5.9) If x =

.

1x; + 1x: + x , , y = C yi + 1y; + y o are the canonical k

k

k

k

decompositions of two vectors x, y of E, then ( u ( x )I y ) =

1k pk(xL I y i ) + 1 k

vk(x;

I yf)

the series on the right-hand side being absolutely convergent (Section 6.4). This formula at once shows that the self-adjoint operator u is positive if and only if there are no negative eigenvalues vk, and that it is nondegenerate if and only if u-'(O) = (0). If u is nondegenerate, and if in each eigenspace E ( I ) (A # 0), we take a Hilbert basis B, (consisting of a finite number of vectors), then the union of the B, is a denumerable set which constitutes a Hilbert basis in E (Section 6.5). (11.5.10) Under the assumptions of (11.5.8), it should be observed that it is quite possible that the self-adjoint compact operator u in E, is nondegenerate, whereas its continuous extension u" to E is degenerate (in other words, the kernel of u is not necessarily dense in the kernel of u"); this may happen even if u is a positive self-adjoint operator.

334

XI

ELEMENTARY SPECTRAL THEORY

For compact self-adjoint operators in a Hilbert space E, (1 1.5.7) yields a formula for the solutions of the equation u(x) - Ax = y in E: (1 1.5.11 ) Let y =

k

y;

+

k

y;

+ y o be the canonical decomposition of y in E.

Then: (a) Zf A is not in sp(u), the unique solution x of the equation u(x) - Ax = y is given by its canonical decomposition (11.5.11.1)

(b) If A is one of the eigenoalues pk (resp. v,), then, in order that the equation u(x) - Ax = y have a solution, it is necessary and sufJjcient that y; = 0 (resp. y; = 0). The soliitions are then given by formula (1 1.5.1 1 . I ) in which the term corresponding to p,, (resp. vk) is replaced by an arbitrary element of E(pk) (resp. E(vk)). In order that the equation u(x) = y haoe a solution, it is necessary and 9 (c) sufJicient that y o = 0 and that the series 2 (1/p;) /Iy; II and ( 1 /v;) II y i [I2 be

convergent; the solutions are then given by (1 1.5.11.2)

x=

c 1 y; + c -1 y; + -

k pk

k

k

Xo

k vk

with xo arbitrary in u-'(0). Results (a) and (b) at once follow from (1 1.5.7) and (1 1.5.6), the formulae being obtained by using the uniqueness of the canonical decomposition. The same argument proves that if there are solutions to u(x) = y , they are necessarily given by (11.5.1 1.2), hence the necessity of the conditions; and if these conditions are satisfied, then the right-hand side of (1 1.5.1 1.2) is an element of E (by Section 6.4) which satisfies u(x) = y .

PROBLEMS

1. Let E be the vector space of all indefinitely differentiable complex valued functions defined in the interval [0, I ] of R (Section 8.12); E is made pto a prehilbert space by the herrnitian form

5

COMPACT OPERATORS IN HILBERT SPACES

335

Let u be the linear mapping of E into itself such that u(x) = ix’. Show that u is selfadjoint, but is not continuous in E. (Consider thesequence (x,) wherex,(t) = (sin nt)/n.) 2. Let F be a separable Hilbert space, (en) (n 2 1) a Hilbert basis of F, u the compact operator in F such that o(eJ = (el e,,)/n (Section 11.2, Problem 3). Let E = o(F), and let u be the restriction of u to E, which is such that u(E) c E. Show that in the prehilbert space E, u is a compact operator which has no adjoint. 3. (a) Let E be a complex Hilbert space, f a continuous hermitian form on E x E; show that there is a constant c such that If(x,y)I S c Ilxll . llyll (cf. (5.5.1)), and show that there exists a unique continuous hermitian operator U in E such that f ( x , Y ) = (UX I Y ) . (b) Suppose E is separable, and let (en)n21be a Hilbert basis for E; let V be

+

the continuous linear operator in E defined by Vel

m

= n= 1

e./n, Vei = 0 for i > 1,

and let W = VV*. Let Eo be the subspace of E consisting of the (finite) linear combinations of the e n ,and let f b e the restriction to Eo x Eo of the mapping (x, y ) + ( Wx I y). Show that f is a continuous hermitian form on Eo x Eo , but that there is no linear operator U in Eo such that f(x, y ) = (Ux I y ) in Eo x Eo . (c) If u is the operator defined in Problem 1, show that the hermitian form (x, y ) + (u(x) 1 y ) is not continuous in E x E. 4. Let E be a complex Hilbert space, u a hermitian operator in E. Prove that u is necessarily continuous. (Assume the contrary, and show that it is possible to define by induction a sequence (x,) of points of E such that /lx. I1 = 1 for every n, and an orthonormal sequence (en) such that: ( I ) x. is orthogonal to u(el), ..., u ( e n - l ) ;(2) if y, is the orthogonal projection of u(x,) on the subspace V, orthogonal to e l , . . , en-I , then

.

llynll>2n3 and IIy,ll 2 2nZ I(u(l$xk/k2)

point x

= n= I

l e n ) l ; (3) en=y,,llynll. Then consider the

x,/n2 in E and obtain a contradiction by showing that I(u(x) I en)I

for every n ; to do this, decompose x into x: f (x,/n2)

2 m

x: =

+ x:,

with

>n

“-1

xi =k = l xk/kZand

xk/kZ,and use throughout the identity (u(y) I z) = ( y I u(z)) (“method of the

k=n+l

gliding hump”).) Compare to Problem 3(c) and to (12.16.7). 5. Let E be a complex prehilbert space; if I!/, V are two hermitian operators in E, we write U 2 V if the hermitian operator U - V is positive, i.e. if (Ux I x ) > (Vx I x) for any x E E. (a) Suppose E is a Hilbert space, and there is a number rn > 0 such that U 2 m . I E . Show that U is a linear homeomorphism of E onto itself. (First remark that IIUxll 2 m IlxIl for any x E E, hence (Problem 4) that U is a linear homeomorphism of E onto a closed subspace M of E; next observe that if a point x E E is orthogonal to M , then x = 0.) (b) Let F be the subspace of the prehilbert space E defined in Problem I , consisting of the restrictions to [0, I ] of all polynomials with complex coefficients. Let U be the operator which associates to any polynomial x E F the polynomial ( I -f- t ) x ( t ) .Show that U is a continuous hermitian operator in F such that U 2 I E , but that U(F) is dense in F and distinct from F. 6. (a) If U is a positive hermitian operator in a complex prehilbert space E, show that, for any x E E, 1lUxIl4 < ( U x I x ) ( U ~ x IUx) (consider the positive hermitian form ( x , y ) ( U x I y ) and use (6.2.1)). --f

336

XI ELEMENTARY SPECTRAL THEORY (b) Suppose in addition U is continuous (cf. Problems 3(c) and 4).Deduce from (a) that llU(I = sup (Ux 1 x ) . IlXll d 1

7. Let F, G be two separable complex Hilbert spaces, (a,) (resp. (b,)) (n > I ) a Hilbert basis of F (resp. G), L the Hilbert sum (Section 6.4) of F and G . Let u be the continuous operator in L defined by u(a,) = 0, u(b,) = aJn, and let E = u(G) u*(u(G)). Let u be the restriction of u to E. Show that u is compact and has an adjoint, but that u* is not compact. (Observe that u(G)is dense in F but not closed in F; if ( x , ) is a bounded sequence of points of u(G)converging to a point in F, but not in u(G),show that the sequence (u*(x.)) converges to a point in L which is not in E, using the fact that the restriction of u* to F is injective.) 8. The notations and assumptions are those of (11.5.7). Let (h.) be the decreasing sequence of numbers >O such that, for each k , the number of indices n such that h, = p x is equal to dim(E(p,)); let (a,) be an orthonormal system in E such that, for the indices n for which h, = pk, the a, constitute a basis of E(pk).We say that (A") is the full sequence of strictly positive eigenvalues of u. (a) Show that h, is the maximum value of (u(x) I x ) when x varies in the subset of E defined by the relations !/xlj= 1, ( x 1 ah) = 0 for 1 < k < n - 1; furthermore, that maximum value is attained for x = a, (use (11.5.7(d))). (b) Let z,, .. ., z , - ~ be arbitrary vectors in E, and denote by pu(zl,.. ., z , , - ~ )the 1.u.b. of (u(x) I x) when x varies in the subset of E defined by the relations jlx/!= I , ( x ( z k )= 0 for 1 < k < n - 1. Show that h, = p,(al,. . ., a n - l )< p,(zl, . . . , z ~ - ~(the ) "maximinimalprinciple"; take x in the subspace generated by a , , . . . , a, and verifying the relations (x I zk)= 0 for 1 < k C n - 1). (c) Let u', u" be two compact self-adjoint operators, and suppose u = u' u"; let (A;), (h:) be the full sequences of strictly positive eigenvalues of u' and u", respectively, (a;) and (a:) the corresponding orthonormal systems. Show that if A ; , h: and hp+q-lare defined, then h,+,-, G h; i- hi (consider p u b ; , . . , a;-,, a ; , . . , u;-~). If the sequence (h:) is finite and has N terms, and if h; and A,+, are defined, then hp+N< hb (same method, observing that (u"(x) 1 x ) < 0 if (x 1 a;) = 0 for 1 < j < N). (d) Under the same assumptions as in (c), show that if hi and A, are defined, then Ih, - h; 0 (resp. U" < 0), then h, h; (resp. h, < hi) (same method). (e) When E is finite dimensional, transcribe the results of (b), (c), (d) for hermitian forms on E x E (see Problem 3). Apply to the following problem: let fi (1 < i < n) be regulated functions in a compact interval I = [ a ,61, and let I' = [c, d ] be an

+

+

.

interval contained in 1; let A

= det(Jbabfi';

d t ) , A'

= det(SdJA c

.

dt) be the Gram

determinants corresponding to I and 1'; show that A' < A, by expressing the Gram determinants as products of eigenvalues. 9. (a) Let u be a compact self-adjoint operator in a complex Hilbert space E. Let H be a closed subspace of E, and p the orthogonal projection of E onto H (Section 6.3). Show that the restriction u to H of p 0 u (or o f p u 0 p ) is compact and self-adjoint and that (u(y)I y ) = (u(y) I y ) for y E H (use the relation p* = p ) . Let (An), (p.) be the full sequences of strictly positive eigenvalues of u and u respectively. Show that if h, and pn are defined, then p. < hn (use Problem 8(b)). (b) Suppose in addition u is positive. Show that for any finite sequence ( x ~Q )k G~n of points of E, det((u(x,) I X I ) ) < hlh, . . . h, det((x, I x J ) ) (apply (a) to the subspace H generated by x1, . . , xJ. 10. (a) Let u be a hermitian operator in a complex prehilbert space E. Show that Il~ (use Cauchyfor any integer n > 0 , and any x E E, llu"(x)112 < l l ~ " - ~ ( x )I!u"+l(x)/l Schwarz). 0

.

5 COMPACT OPERATORS IN HILBERT SPACES

337

+

(b) Suppose E is a Hilbert space, and u is a compact self-adjoint operator. If u(x) 0, show that u"(x) # 0 for any integer n > 0, and that the sequence of positive numbers m, = llu"+'(x)ll/llu"(x)l~ is increasing and tends to a limit, which is equal to the absolute value of an eigenvalue of u. Characterize that eigenvalue in terms of the canonical decomposition of x ; when does the sequence of vectors u"(x)/ llu"(x)11 have a limit in E ? (Use (11.5.7).) 11. Let u be a compact self-adjoint operator in a complex Hilbert space E, and let f b e a complex valued function defined and continuous in the spectrum sp(u). Show that there is a unique continuous operator u such that (with the notations of (11.5.7)), the restriction of u to E(pk) (resp. E(vk), E(0)) is the homothetic mapping y -f(pk)y (resp. y + f ( v , ) y , y + 0). This operator is written f ( u ) ; one has ( f ( u ) ) *=.f(u). If g is a second function continuous in sp(u), and h = f + g (resp. h = f g ) , then h(u) = f ( u ) -t- g(u) (resp. h(u) = f ( u ) g ( u ) ) . In order thatf(u) be self-adjoint (resp. positive and self-adjoint), it is necessary and sufficient that f ( 6 ) be real in sp(u) (resp. f(5) 0 in sp(u)); in order that f ( u ) be compact, it is necessary and sufficient that f ( 0 ) = 0. 12. Let u be a compact positive hermitian operator in a complex Hilbert space E. Show that there exists a unique compact positive hermitian operator u in E such that v2 = u ; v is called the square root of u.

Let E be a separable complex Hilbert space, (e,Jnbl a Hilbert basis of E. Let u be the compact operator in E defined by u ( e l )= 0, u(e.) = e n W l / nfor n > 1 . Show that there exists no continuous operator u in E such that u2 = u. (Observe first that H = u*(E) -is a closed hyperplane orthogonal to el, and that it is contained in H' = u*(E); as H' is orthogonal to x l= u(el), conclude that necessarily x1 = 0; next consider x2 = u(e2), and observe that u(v(e2))= 0, hence necessarily x 2 = hel, where is a scalar; but this implies x2 = 0, hence u(e2)= 0, a contradiction.) 14. Let E be a separable complex Hilbert space, ( e J n b o a Hilbert basis, u the compact positive hermitian operator in E defined by u(eo)= 0, u(e.) = e./n for n > 1 .

13.

The point a =

x (e,/n) does not belong to u(E). Let Eo be the dense subspace of E m

+

n=1

which is the direct sum of u(E) and of the one-dimensional subspace C(eo a ) . Show that the restriction u of u to Eo is a compact positive hermitian operator which is nondegenerate, although its continuous extension d = u to E is degenerate; furthermore, in the canonical decomposition (11.5.8) of the vector eo a E Eo , the summands do not all belong to Eo .

+

Let U be a compact operator in a complex Hilbert space E, and denote by R and L the respective square roots (Problem 12) of the compact positive hermitian operators U * U and UU*,respectively. Show that there exists a unique - continuous operator V in E, whose restriction to F = R(E) is an isometry onto U(E), whose restriction to the orthogonal supplement F' to F is 0, and which is such that U = VR (observe that llUxll= l l R x l l f o r e a c h x ~ E ) . P r o v ethat R = V * U = R V * V , a n d L = VRV*. (b) Let (m,) the full sequence of strictly positive eigenvalues of R, and (a,) a corresponding orthonormal system (Problem 8). If h, = Va,, show that (b.) is a n orthonormal system, and that, for any x E E, Ux - = x m . ( x I a&, where the series on the

15. (a)

right-hand side is convergent (if R,x (11.5.7), that lim IlR n-r m

-

"

= k= 1

"

mA(xIak)ak, show, using the proof of

R,I/ = 0, and apply (a)). Deduce from that result that (cL.)

is also the full sequence of strictly positive eigenvalues of L , and that (b,) is a corresponding orthonormal system. The sequence (m,) is also called the full sequence of singular values of U.

338

XI

ELEMENTARY SPECTRAL THEORY

(c) Let (p.) be the sequence of distinct eigenvalues # 0 of U,arranged in such an order that 1p.l > (pn+llfor every n for which is defined; let d, be the dimension of N(pn), and let (h.) be a sequence such that h, = pI,Ih,( > lh,,+ll for every n for which is defined, and for each k for which px is defined, the indices n for which h, = pk form an interval of N having dx elements. Show that, for each index n such that h,, and a, are defined, N(pk)for 1

< k < r , and

n (h,I < n a , . (Let V be the (direct) sum of the subspaces n

I= I

i=1

let Uv be the restriction of U to V; show that there is in V a Hilbert basis (eJ)lsJs, such that (U(eJ)Ie x )= 0 for k >j ; for n < m, if W, is the subspace of V having e l , ..., en as a basis, let U. be the restriction of U to W,, and let P,,be the orthogonal projection of E on W,. Show that

n lhjI2 is

j= 1

equal to the determinant of U ~ U=, P, U *UP,, and apply Problem 9(a).) (d) Let T be an arbitrary continuous operator in E, and let (y,,) (resp. (6,)) be the full sequence of singular values of UT (resp. T U ) . Show that y. < a. IITll (resp. 6, < a. IITll) for all values of n for which a,, y. and 6, are defined (if S = TU, observe that S * S < IITIIZU*Uand use Problem 8(d)). (e) Suppose T is also a compact operator, and let (fin) be the full sequence of its singular values. Show that n" y J < J=l

(n" a,)( fi6,) J=1

J=I

for all values of n for which

a,, fin, and ynare defined (apply Problem 9(b)). 16. Let E be a complex Hilbert space, (an)a sequence of points of E, (A,) a sequence of real numbers. Show that if the series u(x) = h,(x I a&. is convergent in E for n

every x E E, u is a hermitian operator in E. The convergence condition is always satisfied if the series of general term h,l~anllzis absolutely convergent. If in addition (a,) is a n orthonormal system, the convergence condition is satisfied if the sequence (A,) is bounded. When the h. are 2 0 and the convergence condition is satisfied, u is a positive hermitian operator. 17. Let E be a complex Hilbert space, Eo a dense vector subspace of E; ( x l y ) and //XI/ denote the scalar product and the norm in E. Suppose a second norm //xllois given on E,, for which E, is a Banach space, and is such that, for x E E,, one has llxll < a .Ilxllo, where a is a constant (in other words, the identity mapping lE0 of Eo with the norm /lxllo,into Eo with the norm IlxlI, is continuous). (a) Let U be a hermitian operator in Eo (which a priori is not assumed to be continuous); show that if U is continuous for the norm llxllo, it is also continuous for the norm llxll. (If I/U//o is the norm in 2'(EO) when Eo is given the norm Ilxll,, show that, for every integer n , one has, for x E Eo ,

Use the inequality IIUkxllz< llxll. IIU2k~II(Problem lO(a)).) (b) Let 0 be the continuous extension of U to E, which is a hermitian operator in E. Show that the spectrum of 0 is contained inthespectrumof U(when U isconsideredas an endomorphism of the Banach space Eo for the norm llxllo). (If 5 is a regular value for U,observe that ( U - 5 . l E 0 ) - I can be extended by continuity to E, using (a).) (c) Every eigenvalue h of U is real; if V = U - A . lE0, and if E(h)= V - ' ( O ) is finite dimensional in Eo, V(E,) is supplementary and orthogonal to E(A) in the prehilbert space Eo, and in addition it is closed for the norm llxll,; it follows from (12.16.8) that the restriction of V to V(E,) = F(h) is a linear homeomorphism of

5

COMPACT OPERATORS

IN

339

HILBERT SPACES

F(h) onto itself for the norm I/x/Io.Show that if p = 0 - h . l,, then P-l(O) = E(h) and P(E) = in E (apply (a) to the inverse of the restriction of V to F(h)). (d) Deduce from (c) that if U (or a power of U ) is a compact operator in Eo (for the norm Ilxllo),then 0 is a compact operator. (If (A,) is the sequence of eigenvaluesof U, deduce from (b) and (c) that the subspace of E orthogonal to all the subspaces E(h,) is the kernel of 0.) 18. Let E be an infinite dimensional complex Hilbert space. For a positive hermitian operator T i n E, the following conditions are equivalent: (1) T(E) is dense in E; (2) T-'(O) = {O}; (3) (Tx I x) > 0 for any x # 0 (use the Cauchy-Schwarz inequality applied to (TxJy)); (4) Tis nondegenerate. We say that a continuous operator Uin E is quasi-hermitian if there exists a nondegenerate positive hermitian operator T such that TU= U*T. (a) Show that every eigenvalue of U is real; if V = U - h . l E and if V-'(O) is finite dimensional, then E is a direct topological sum of V(E) and V-'(O), and h is a simple pole of (U - 5 . lE)-'. (Consider the Hilbert space obtained by completing E for the scalar product (Tx I y ) , and apply Problem 17.) a: = 1. (b) Let (01") be an infinite sequence of distinct real numbers such that

"

Let E be the Hilbert sum of a sequence of finite dimensional Hilbert spaces En such that dim(E,) = n (n 2 1). In En, let ( e i J l be a Hilbert basis, U, an operator in En such that Uneln= cciein ei+,,"for i < n - 1 and U,e,, = tL,e,, . Prove that llUnl.il< 2, but for any complex number 5 such that 151 = 1, ll(U, 5 . lEn)-'ll 2 44;. There is a unique continuous operator U on E whose restriction to each En is Un; show that U is quasi-hermitian and that the CL, are its eigenvalues, but that its spectrum contains the circle 151 = 1. (c) Let U be a compact quasi-hermitian operator. Prove that if U # 0, the spectrum of U cannot be reduced to the point 0, the eigenvalues hn # 0 of U are simple poles of (U- 5 . I,)-' and N(h,; U )= E(&; U ) and T(E(h,; U ) )= E(h.; U *). Furthermore, the intersection of the subspaces F(h,; U ) is equal to U -'(O), and the intersection of U-'(O) and U(E) is reduced to 0 (method of (a)). (d) Suppose E is separable, and let U be a compact continuous operator in E having the following property: there is a sequence (h,) of real eigenvalues of U * such that the sum of the E(X,; U * ) is dense in E. Prove that U is quasi-hermitian. (Show that (for a suitable k(n)), and there is in E a total sequence (b.) such that U*b, = hkCn)bn a,(x I b,)b, with suitable a. > 0; cf. Problem 16.) define T such that Tx

+

+

=I n

(e) Suppose E is separable, and let U be a compact operator in E satisfying the following conditions: (1) all the eigenvalues An of U are real and k@.; U )= I for every n ; (2) the intersection of the subspaces F(hn;U)is equal to U-'(O); (3) the intersection of U -I(O) and U(E) is (0). Prove that U is quasi-hermitian (use (11.5.5) and (d)). (f) Suppose E is separable, and let (eJnbobe a Hilbert basis for E. Prove that the operator U defined by Ue2.

= 0,

Ue2,+

=

nfl

+

for all n 2 0, is compact and quasi-hermitian, but the sum U -I(O) U(E) (which is an algebraic direct sum) is not a topological direct sum (cf. Section 6.5, Problem 2). m

(g) With the same notations as in (f), let U be the operator defined by Ueo =

e,/n,

"=I

Ue, = e,/nZ for n 2 1 ; using (e), show that U is compact and quasi-hermitian, but the sum U-'(O) U(E) is not dense in E; conclude that U * is not quasi-hermitian.

+

340

XI ELEMENTARY SPECTRAL THEORY

19. Let E be a Hilbert space, U a continuous operator in E. Suppose there is an element n f 0 in E such that: ( I ) the elements a, = U"a for n > 0 (with Uon = a ) form a total sequence in E; (2) the image of E by the hermitian operator V = U U * is the onedimensional subspace D = K a . Let Eo be a closed vector subspace of E not reduced to 0 and such that U(Eo) c Eo; let Uo be the restriction of U to Eo , and Po the orthogonal projection of E onto Eo . (a) Show that Eo cannot be orthogonal to D . (Observe that for any y orthogonal to D, U y = - U * y , and conclude that if Eo was orthogonal to D, one would have U p = (- I)"U*"yfor every y E Eo; show that this contradicts the assumption that the U"n form a total sequence in E.) (b) Prove that for any x E Eo , U:x = Po U,*x = Po U * X . (c) Prove that the image of Eo by Vo = Uo U z is not reduced to 0, hence is the subspace Do = Po(D) of dimension 1 (use the fact that Poa # 0 and the result of (b)). (d) Prove that the elements U;(Pon)constitute a total sequence in Eo. (Let Fo be the closed vector subspace of Eo generated by that sequence, and F6 the orthogonal supplement of Fo in Eo. Prove that FL is orthogonal to D and that Uo(Fb) = U(Fb) = Fb; conclude as in (a).) 20. Let E be a separable Hilbert space, U a compact operator in E whose spectrum consists of 0 and of an infinite sequence (AJof distinct eigenvalues # 0 such that k(h,) = 1 and that E(hJ is one dimensional for every n. For each n , let a. be an Figenvector of U corresponding to h,, and b, an eigenvalue of U * corresponding to h, (see (11.5.5)); a. and b, are chosen in such a way that (a. I b,,) = I . (a) Let A and B be the closed vector subspaces of E respectively generated by the a. and the b,, B' and A' the orthogonal supplements of B and A, respectively. Show that B' is stable for U , contains U - l ( O ) and that the restriction of U to B' has a spectrum reduced to 0. (b) Suppose the series of general term lh,, . lla,ll~ IIb.11 is convergent. Show then h,(x I b.)a,, where the series is convergent in E; U - l ( O ) that for any x E A, U X

+

+

=c "

then contains A n B'. (c) Give an example in which A n B' is infinite dimensional and U ( A n B') = A n B'. The following method may be used: let E be a Hilbert sum of three Hilbert spaces of infinite dimension F, R, S, having respective Hilbert bases (f"),(r,,), and (.yn); define a sequence (an)in F -1 R such that f. is the projection of a, in F and the closed vector subspace generated by the a, is F t R. To do this, observe that in a Hilbert , closed vector subspace generated by the space H with a Hilbert basis ( e n ) n 3 othe eo en for n > I is equal to H, and take for F R the Hilbert sum of a sequence o f Hilbert spaces all equal to H. Define U such that Unn= h,,a,,, Urn= p,,r,,-l for I? 2 I , Url = 0, Us,, = p , , ~ . + ~ where , the sequences (ti,) and (pn)converge rapidly enough to 0 (cf. Section 11.2, Problem 3). One thus gets B = F, A = F I R, and A n B'= R.

+

+

6. T H E FREDHOLM INTEGRAL E Q U A T I O N

We now apply the preceding theory to the example (11.2.8). We consider here the prehilbert space G of continuous complex-valued functions in I = [a, h], with ( f i g ) =Jabf(t)z) rlt, and the operator U such that U f is the function

6 THE FREDHOLM INTEGRAL EQUATION

341

t -+ @ K ( s , t)f(s) ds.

We say that the operator U is defined by the kernel function K. (11.6.1 ) The compact operator U in G has a compact adjoint which is defined by the kernel function K* such that K*(s, r ) = K ( t , s).

We prove for a d x

< b the identity

which, for x = 6,will yield the result by definition. Both sides of (11.6.1.1) are differentiable functions of x in [a,b ] , by (8.7.3) and Leibniz's rule (8.11.2); they vanish for x = a, and their derivatives are equal at each x E [a, b ] by (8.7.3) and (8.1 1.2), hence they are equal everywhere in [a, b ] (8.6.1 ). We leave to the reader the expression of the criterion (1 1.5.6) for that particular case (the " Fredholm alternative ").

If K ( t , s) = K(s, t ) (in which case the kernel K is called hermitian), the compact operator U is self-adjoint. As the prehilbert space G is separable ((7.4.3) or (7.4.4)), it can be considered as a dense subspace of a Hilbert space (6.6.2), and therefore we can apply to the operator U the results of (11.5.8). We shall denote by (An) the sequence of the (positive or negative) eigenvalues of U, each being repeated a number of times equal to its mu/tiplicity, and ordered in such a way that lAnl b / A n + , [ ; and we will denote by (q,) an orthonormal system in G such that, if the values of n for which An = pk (resp. A,,= v k ) are ni, m + I , . . . , m + r, then q m ,v",+~, . ..,qm+r constitute a basis for the eigenspace E(pk) (resp. E(vk)); we therefore have U ( q , ) = Anqnfor each t i . The q, are called eigenfunctions of the kernel K.

e

(11.6.2)

If K

is a hermitian kernel, the series

c A; d [ d t n

1 A:

is conwrgent and

n

jabIK(&t)12 d s .

a

Indeed, if we apply the Bessel inequality (6.5.2) to the function s -+ K(s, t ) and to the orthonormal system ((p"), we obtain, for any N

n= 1

342

XI

ELEMENTARY SPECTRAL THEORY

i.e.

c 2: N

(11.6.2.1)

fl=

1

lcpn(t)I2

G [IK(s9 t>I2 ds

for every t E I . Integrating both sides in I and using the relations (cp, and (11.6.1 .I) yields the result. The canonical decomposition in be written f

=

c c,

cpn

n

+f o ,

e of any function f

where c,

=

( f I cp,)

I cp,)

1

G (11.5.7) can here f(t)cp,(t) d t ; but, as

=la b

=

E

already observed,,f, may fail to be in G; on the other hand, the series

e,

1 cncpn

converges in the Hilbert space but not in general in the Banach space E = %?,-(I) (the series cncpn(t)will not necessarily converge for every t E I).

1 n

However: (11.6.3)

f (t)

If K is a hermitian kernel, and .f = Ug f o r a function g E G (i.e. K(s, t ) g ( s )ds), then rhe series c, cp,(t) converges absolutely and

=lab

uniformly

tof(t)

We have in

1 n

in 1.

c the canonical decomposition g = 1dncpn + g o ; as

V is a

n

continuous linear mapping of G into E = %?,-(I) (11.2.8), U can be extended to a continuous linear mapping of into E, and Ug, = 0, hence we have -f = Ug = Andncpn,where now the convergence is in E; i.e. the series

1

e

A,, d, cp,(t) converges uniformly to f ( t ) in E; as c, = (fI cp,)

= (Ug I cp,)

=

n

( g I Ucp,) = 1,(g I cp,) = And, , we have proved (11.6.3) except for the statement on absolute convergence. But for any integer N, we have, by Cauchy-Schwarz (for finite dimensional spaces)

and the right-hand side is bounded by a number independent of N, by Bessel's inequality (6.5.2) and (11.6.2.1). (11.6.4) I f K is hermitian, and 1 # 0 is not in the spectrum of U, the unique solution f'of the equation Uf - Af = g , f o r any g E G, is such that

where the series is absolutely and uniformly convergent in I, and dn = ( g I cp,).

6 THE FREDHOLM INTEGRAL EQUATION

343

e

We know that the unique solution of U f - Af = g in belongs to G since G (11.5.6), and by (11.5.11) we have c, = (fl cp,) = l/(A,, - 2). As g /If= Uh we can apply (1 1.6.3), and this proves the result. g

E

+

(11.6.5) Under the same assurnptions us in (11.6.4), the unique solution of U f - /If = g can be written

with

where the series is absolutely and unifornily coniIergerit for (s, t ) E I x I. By the proof of (11.6.3), we have

An dncp,,(t) = Q ( t ) ,the series convergn

ing absolutely and uniformly in I. As 1

A(An - A)

+ -1= A2

An

A(Ai -A)

the formula in (11.6.4) gives

The theorem will follow when we have proved the uniform convergence of the series A: Icpn(s)12: for there is a 6 > 0 such that IA, - A1 2 6 for each n, fl

hence

by Cauchy-Schwarz, and this will prove that the series

is absolutely and uniformly convergent in I x I ; the conclusion then results from (8.7.8).

344

XI ELEMENTARY SPECTRAL THEORY

Now consider the function H(s, t )

=I

b

K ( u , s ) K ( t , u ) du; for each fixed

I~qn(s)qn(t)

t E I, we can apply to it (11.6.3), and we see that H(s, t ) = n

where the series is convergent for any pair (s, t ) E I x I. In particular H(s, s ) = il: Iqn(s)12 for all s E I, and H(s, s) is continuous; by Dini’s n

theorem (7.2.2), the convergence is uniform in I. Q.E.D.

(11.6.6)

If K is hermitian, then

uniformly,for s E I.

With the notations of the proof of (11.6.5), we have (11.6.6.1) n-tm

\

k= 1

uniformly for s E I ; if we evaluate the integral in the statement of (1 1.6.6), using the fact that the qkare eigenvectors of U , and that they are orthogonal, we obtain the expression in the left-hand side of (11.6.6.1), whence the result. -

In general, the series

d,cp,(s)q,(t)

wiI/ not be contlergent for all

n

(s, t ) E 1 x I ; but we have the special result:

(11.6.7) (Mercer’s theorem) Suppose the coinpact operator Udejiied 61) the hertnitian kernel K(s, t ) is posititre. Then we have K ( s , t ) = 1, q,(s)q,,(t), n

where the series is absolutely and uniformly coniiergent in I x I.

We recall that we have here I n> 0 for every n (11.5.9). We first prove that for each s E I , the series I nlqn(s)I2 is convergent. For any s E I, we have K ( s , s) > 0. Otherwise, there would exist a neighborhood V of s in I such that B(K(s‘, t ) ) Q - 6 < 0 for (s’, t ) E V x V. Let q be a continuous mapping of I into [0, 11, equal to 1 at the points, to 0 in I - V (4.5.2). Then we have

6 THE FREDHOLM INTEGRAL EQUATION

345

by (8.5.3). But the left-hand side is (Ucp I cp), and this violates the assumption that U is a positive operator. Remark now that for any finite number of eigenvalues Ak (1 < k d n ) ~ , ( s , t ) = ~ ( s t, ) -

&(Pk(S)fpk(t)is the kernel function of a positive operk= 1

ator U,,, for we have

(un

f

f 1 f) = (uf I f) -k = 1 I(f I ‘Pk)12;

but the right-hand side of that equation can be written (Ug Ig) with fl

g=f-

k= 1

(fI (Pk)(Pk,as is readily verified, hence is positive by assumption.

Therefore, by (5.3.1) it follows from K,(s, s) 2 0 that the series is convergent, and we have

c A,,

Icpn(s)12

1 A,, lcpn(s)12

,< K(s, s) for all s E I. By Cauchy-

n

Schwarz, we conclude that

for all (s, t ) E I x I. Hence, as K(t, t ) is bounded in I, f o r f i x e d s E I, the series A,,cp,(s)cp,,(t)is uiiifornily conrergent for t E I. By (11.6.6), (8.7.8), and

1

-

fl

An cp,I(s)cp,,(t) = K(s, t ) for all (s, t ) E I x I since

(8.5.3), we conclude that n

t -+ IK(s, t ) -

2

1A , l a ) c p n ( t ) l fl

particular, we have K(s, s) = series

1 A,, lq,(~)1~is n

proves that the series

is continuous in I and its integral in I is 0. In

c An

Icp,,(~)1~; by Dini’s theorem (7.2.2) the

n

therefore uniformly contiergent in I, and (11.6.7.1) -

1 A,, cp,(s)cp,(t)

is absolutely and uniformly convergent

n

in I x I, which ends the proof.

Remarks (11.6.8) The result (11.6.7) is still true when we only suppose that U has a finite number of eigenvalues vk < 0 (1 < k < m).For (11.5.7(c)) shows then that in the space FL+,,orthogonal supplement of E(v,) -.. E(v,) in G, the restriction of the operator U is positive, and we apply (11.6.7) to that

+ +

346

XI

ELEMENTARY SPECTRAL THEORY

operator, which, as is readily verified, corresponds to the kernel function K(s, t ) - A,,q h ( ~ ) q h (where t), h runs through all the indices (in finite num-

1 h

ber) such that Ah < 0. The conclusion is then immediate. (11.6.9) We can consider the operator U in a larger prehilbert space, namely the space F, of regulated functions (Section 7.6) which are continuous on the right (i.e. such thatf(t+) = f ( t ) for a < t < b) and such that f ( b ) = 0; for such a function the relation Jab If(t)12 dt = 0 implies f ( t ) = 0 everywhere in 1 = [a, b], for it implies f(t) = 0 except at the points of a denumerable subset D (by (8.5.3)), and every t such that a < t < b is limit of a decreasing sequence of points of I - D. The space G may be identified to a subspace of F, , by changing eventually the value of a continuous function f E G at the point 6; it is easily proved (using (7.6.1)) that G is dense in F, . The argument of (11.2.8) then shows that U is a compact mapping of F, into the Banach space E = gC(I) (and a fortiori a compact mapping of the prehilbert space F, into itself). All the results proved for the operator U in G are still valid (with their proofs) when G is replaced by F, .

PROBLEMS 1. Extend the results of Section 11.6 (with the exception of (1 1.6.7)) to the case in which K(s, t ) satisfies the assumptions of Section 8.1 1, Problem 4 (use that problem, as well as Section 11.2, Problem 5). 2. In the prehilbert space G of Section 11.6, let (J,) be a total orthonormal system (Section 6.5); let

KAs, t )

=

kh(slz)

k= 1

and

HAS) = /ablK.(s, t)l df

(the “nth Lebesgue function” of the orthonormal system (fn)).For any function g E G,

let s,(g)

n

=

(g I f x ) f x , so that s,(g)(x) =

k= 1

1.”

K,(x,

t ) g ( r ) dt

for any x

E I.

(a) Prove that if, for an x o E I, the sequence (H,(xo)) is unbounded, then there exists a function g E G such that the sequence (s,,(g)(xo))is unbounded. (Use contradiction, and show that under the contrary assumption it is possible to define a strictly increasing sequence of integers (n,), and a sequence (g,) of functions of G, with the following properties: (1) let ch = sup IJ:K.(xo,

“

assumption), let dk = c1 -1 c2 sup(ml,. . ., mk-,); then m,

t)gh(t) dt

1

(a number which is finite by

+ . . . + ck-1, let mk ~Kn,(xo,t ) dt, and let qr 2,+l(9, + l)(d, + k ) ; (2) let vkbe a continuous func=

nl

=

Jnu

I

, dt tion such that vk(a)= Tt(b) = 0, Ivk(t)l< I in I and J o b ~ . , ( x o t)Tk(f)

I

mk/2

(see Section 8.7, Problem 8); then g k = ~ , / ( 2 ~-t ( 9I)).~ Then show that the function m

g=

k=l

g k is continuous in 1 and contradicts the assumption: to evaluate the integral

6 THE FREDHOLM INTEGRAL EQUATION

gi

Knk(xO,t ) g ( t ) dt, split g into i
347

+ ge t- C gi, minorize the second integral and i>h

majorize the two other ones (“method of the gliding hump”).) (b) Show that for the trigonometric system (Section 6.5) in I = [-1, 11, the nth Lebesgue function is a constant h, , and that lim h, = w (observe that

I I

n- m

/A/‘“

(h-l)ln

for 2 < k

+

sin n v t 2 dt 3 sin vt kn

< n). Conclude that, for any xo E I, there exists a continuous function g in I,

such that g(- 1 ) = g(l) = 0, for which the partial sums

.

5 (1’g ( t ) e - i K ” r

h=-n

dt)etkff”/2

-1

of the “Fourier series” of g are unbounded for x = xo (Cf. Section 13.17, Problem 2.) 3. Let g be a continuous complex valued function defined in I = [- 1 , I ] and such that g(- 1) = g(1) = 0; g is extended to a continuous function of period 2 in R. Let K(s, t ) be the restriction ofg(s - t ) to I x I ; ifg(-t) =g(t), the compact operator U defined by the kernel function K(s, I ) is self-adjoint. Show that the functions ~ “ ( t= ) enntt/2/i are eigenvectors of U , the corresponding eigenvalue being the “Fourier coefficient ” a.

= J : l

g(t)e-“”“ dt of g.

Using that result and Problem 2, give examples of a hermitian kernel function K for which the series of general term h , a v . ( t ) has unbounded partial sums for certain values of s and t , and of a positive hermitian kernel function K for which there is a function f e G such that the series

rn

C (fI vn)vn(t) has unbounded partial sums for

fl=l

4.

certain values o f t . Let I = [-2v, 271, and define K(s, t ) in I x I to be equal to the absolutely convergent -sin ns . sin nt for 0 C s < 2n, 0 C t < 2 7 , and to 0 for other values of (s, t ) .=inz in I x I. Give an example of a functionfg G such that in the canonical decomposition off, fo does not belong to G. (The eigenfunctions of K are the functions pnsuch that v,(t)= 0 for -2v < t < 0, y,(t) = n-1’2sin nt for 0 C t < 2n. Take forfa continuous

series

function in I equal to 2 n - t in [0,27r], and show that the series verges everywhere in I, but has a discontinuous sum.)

m

C (f1 ~.)v,(t) con-

n=1

5. With the general notations of Section 11.6, let K be a hermitian kernel defined in

I x I, and let U be the corresponding self-adjoint compact operator in G. Show that for every h > 0, Uhcorrespondsto the hermitian kernel Kh,which is defined inductively by K 1 = K, and Kh(s, t )

=jab w

u)K(u, t ) du.

-

Prove that for h 3 2, Kh(s,t ) = C ht v n ( s ) v n ( t ) ,the series being absolutely and

”= 1

uniformly convergent in I x I. Show in addition that

and that the sequence (Ahfl/Ah)is increasing, and has a limit equal to lh1)’, where h, is an eigenvalue of K of maximum absolute value (use Cauchy-Schwarz).

348

XI ELEMENTARY SPECTRAL T H E O R Y

K be an arbitrary continuous kernel function in I x I, and let U be the corresponding compact operator in G . Let M be a finite dimensional subspace of G such that U(M) c M ; let ( + h ) l d h < n be an orthonormal

6. With the notations of Section 11.6, let

basis of the space M, and write

u+h

=

C

k=l

ahk

4 k . Show that

(For each f E I, apply Bessel's inequality (6.5.2) to the function s +K(s, t ) and the orthonormal system ( # h ) in G.) Let (A,) be the sequence defined (for the operator U ) in Section 11 -5,Problem 15(c). Prove that the series

m

"=l

Ih,12 is convergent, and

(Apply the preceding result to any sum of subspaces N(pk), with the notations of Section 11.5, Problem 15(c).) 7. Give an example of an hermitian kernel K(s, t ) , such that, if U is the corresponding compact operator in G , and V the square root of U 2 (Section 11.5, Problem 12), there is no hermitian kernel to which corresponds the compact operator V . (If there existed such a kernel, Mercer's theorem (11.6.7) could be applied to i t ; take then for K the first example in Problem 3.) 8. In order that the compact operator U defined by an hermitian kernel K(s, t ) be positive, show that a necessary and sufficient condition is that K be (in I x I ) a function of positive type (Section 6.3, Problem 4; to prove that the condition is necessary, write the inequality

JabfT dt K(s, t ) f (s) ds 3 0 Jab

for a function f which is 0 outside arbitrary small neighborhoods of a finite number of points x i of 1 (1 ,< i < n). To prove that the condition is sufficient, use the same method as in Section 8.7, Problem I). Conclude from that property and from Section 6.3, Problem 8 and Section 6.6, Problem 5 , a new proof of Mercer's theorem. 9. (a) A kernel function K(s, t ) defined in 1 x I (with I = [a, b ] ) and satisfying the assumptions of Section 8.1 1, Problem 4, is called a Volterra kernel if K(s, t ) = 0 for s> t . Let M = sup IK(s, t ) l . If U is the compact operator in G corresponding (S,

¶)El

Y

I

to K (Problem l), show that U" corresponds to a Volterra kernel K, such that (Kn(s,f ) l < M"(t - s)"-'/(n - l ) ! for n > 1 and s < t (use induction on n). Deduce from that result that the spectrum of U is reduced to 0, and that for any 4 E C , ll(lE - [ U ) - ' - lE1I < M1 1. Show that for that kernel, the function R(s, 1 ; A) in (11.6.5) is equal to exp((t - s)/A) for s < t , and to 0 for s > t. (Use (8.14.2) to compute U".) 10. Let F, be the prehilbert space defined in (11.6.9) for the interval I = [0, I ] , and let U be the operator defined in Problem 9(b), so that for every function x E F,, y = Ux is the function t - t

Jot

x(s)ds. The space

F+ is a dense subspace in a Hilbert space E

(6.6.2); U is extended by continuity to a compact operator in E, again written U (11.2.9). The closed subspaces Eo of E, distinct from 0 and E, and such that U(E,) c Eo

will be determined in this problem.

7 THE STURM-LIOUVILLE PROBLEM

349

(a) Show (using (7.4.1)) that the operator U satisfies the condition of Section 11.5, Problem 19, with a equal to the constant function of value I in 1; let a. E E, be such that Po a = so a. with so > 0 and I/ao11 =- 1 , so that 0 < so < 1. For every x E Eo , V O X= sb(x1 ao)ao(notations of Section 11.5, Problem 19). (b) For every 5 E C not 0, let f(5) = 1 - si((Uo 5f)-'ao I a,). Prove that for ( # 0, -

f(-<)f(<)

=

I . (To compute that product, use the relation Vo(U0 t- 51)-'ao

= Sb((U0

I- 5 W ' a o

I a0)ao

deduced from (a), and transform the expression

(U8 - { I ) - l ( U o

+ uau, - t ~

{/)-I.)

Deduce from that relation and from Problem 9(a) that f(5-I) is an entire function with no zeros in C. Using the majoration of Problem 9(a), and Section (10.2), Problem 8(c), conclude that f(5) = exp(si{). (c) Conclude from (b) and from Problem 9(b) that no is the function equal to 0 for 0 < t < 1 - si, to I for 1 sb < t d 1 , and that Eo is the closure in E of the subspace generated by the functions of F, which are 0 in the interval 0 < t < 1 - so'. 11. For any two real continuous functionsf, g defined in [0, I], notef* g the function h -

defined in [0, I ] by h ( x )

JOX

f(x

:

-

y)g(y) dy. Prove that

f* g is continuous and equal

to g *f;if f l , f 2 , f 3 are three continuous functions defined in [O, I], prove that * f2) * f3 =- f i * (f2 * f3), which is written f,* f 2 * f3 . For every integer n > 0, one thenf*" *f*"=f*("'+"). defines by induction f*" byf*" =f* (f*("-"); Suppose f is continuous in [0, 11 and f(0) # 0. Prove that if, for a continuous function y defined in [O,I ] , f * g = 0, then g = 0 ("Titchrnarsh's theorem"). Observe that, with the notations of Problem 10, U f = a * L and conclude from Problem 10 that there exists s such that 0 < s < 1 and that the closure in E of the subspace generated byfand the U"ffor n 1 is also the closure of the set of continuous functions which are 0 in [0, s]; the assumption f(0) Z 0 implies s = 0. Observe finally that t +g(l t ) belongs to E and is orthogonal to every U"f for n 2 0. (fl

-

7. T H E STURM-LIOUVILLE PROBLEM

We consider, in a compact interval I = [a, b ] of R, a linear diferential equation of the second order (1 1.7.1 )

Y" - q ( 4 Y

+ AY =f (4

where q(x) is a real-ralued continuous function in I, f ( x ) a complex-valued regulated function in I , which is continuous except at a $finite number of interior points, and A a complex number. By a solution of (11.7.1) is meant a continuously differentiable complex-valued function y ( x ) , such that y'(x) is the primitive of a regulated function with only finitely many discontinuities, and that the relation Y"(X) - q(xly(x) +

=f(4

350

XI ELEMENTARY SPECTRAL THEORY

holds in the complement in f of a j n i t e subset of f . The Sturm-Lioucille problem consists in finding solutions which also satisfy the two boundary conditions

+

12, y(b)

h , y ( ~ ) k,y’(a) = 0,

(1 1.7.2)

+ k,y’(b) = 0

where h,, k , , h , , k , are real numbers, and h i , k i are not both 0 ( i = I , 2). We assume in the following the elementary theory of linear differential equations (see Section 10.8). We first consider the homogeneous equation y” - q(x)y

(1 1.7.3)

+ Ay = 0.

Note that y” is continuous i n I for any solution of (1 1.7.3). (1 1.7.4) There exists a number r > 0 such that,,for A real and < - r, the oiily solution of’(11.7.3) satisfying the boundary conditions (11.7.2) is 0. As q, 2, the h i , k i are real, it is clear that if a solution of (11.7.3) verifies (1 1.7.2), its real and imaginary parts are also solutions verifying the same boundary conditions; we therefore can restrict ourselves to real solutions. Suppose first &at k,k2 # 0, so that we can suppose k , = k , = - 1. Then we can also suppose y(a) # 0, otherwise we would have y‘(a)= 0, and by the existence theorem, our theorem would be proved. Multiplying y by a suitable constant, we may therefore assume that y(a) = 1, y’(a) = h,. Note that if we put z = y’/y for y ( x ) # 0, we have

z’

(11.7.4.1)

Let

= q(x) - 3,

M = sup Iq(x)l, and suppose A xcl

- z2.

< -M

- h:

- 1 ; then we have

z’(a) = q(a) - 3, - h: 2 1, and therefore z is strictly increasing in a neighborhood of a in 1. I claim that y(x) # 0 in I and that z(x) > h, for all x > a. Suppose first that y ( x ) vanishes in I , and let x1 be the smallest solution > a of y(x) = 0. Then y ( x ) > 0 for a < x < x,, hence y ’ ( x l ) < 0 (it cannot be 0, or else y would be identically 0 in I) and as q(x) - 3, > 0 for all x E I, y”(x) > 0 for a < x < x, by (11.7.3), hence y’ is increasing for a < x < xl, and therefore is < O in that interval; it follows that when x < x, tends to x,, z(x) would tend to - co. As z is continuous for a < x < x , , there would be in that interval a smallest x 2 > a such that z ( s 2 ) = h, and f i l < z(x) for a < x < x2. This implies that z’(x,) < 0; but we have z’(x,) = q ( x 2 )- I - 11: 3 1 and we obtain a contradiction, which proves both our assertions. I n a similar way, we see that if we have I d - M - 11: - I , then z(x) d I?, in I . The function z would thus be such that Iz(x)l < c = sup( Ih, 1, 111, I) in I , where c is independent of A. Now from (11.7.4.1) we deduce z’(x) 3 - M - A - c2 = p , hence, by the mean value theorem, h , - h, = z(b) - z(a) 2 p(b - a). If we take A such that

7 THE STURM-LIOUVILLE PROBLEM

351

we obtain a contradiction, and this ends the proof of (11.7.4), when k,k, # 0. Suppose next k, = 0, k, # 0 (hence we can suppose k, = - 1). We can now, by multiplying y with a suitable constant, suppose y(a) = 0, y’(a) = 1 ; then z tends to + co when x > a tends to a. Suppose 1 < - M - 2; then I claim first that y’(x) 3 1 in 1. As y”(x) > 0 by (11.7.3) for x > a in a neighborhood of a, we have y ’ ( x ) > 1 for x > a in that neighborhood. Suppose that y’(x) = 1 for some x > a, and let x, be the smallest solution of that equation. Then y’(x) 3 I for a < x < xl, hence y(x) > 0 in that interval, and y”(x) > 0 by (11.7.3); but we should have y”(xl)< 0, which is a contradiction. We thus see that y is strictly increasing in I, hence z is finite for a < x < b. I claim that z(x) > ( - M - I - 1)1/2 ; otherwise, there would be a smallest x, such that z ( x z )= (-M - I and at that point we would have z’(x,) < 0. But from (11.7.4.1) we deduce z’(x,) 3 - M - 1 - z 2 ( x 2 )> 1, and we have again reached a contradiction. If now we suppose 1 taken such that hi < - M - I - 1 , we find that the relation z(b) = h, is impossible, hence the theorem is proved in that case. The case k, = 0, k , # 0 is treated similarly. Finally, if k, = k, = 0, we may again suppose y(a) = 0, $(a) = 1, and the preceding argument shows that y is strictly increasing in I as soon as I < - M - 2; this of course is in contradiction with the condition y(b) = 0, and the proof is complete.

+

+

Replacing if necessary y(x) by y(x) r, and 1 by I r, we can from now on suppose that there is no notitriilial solutioti of (11.7.3) satisfying both boundary conditions (11.7.2), for 1 < 0. We will use the following identity (11.7.5)

[Qb(u‘ro- u”u) dt = (u’(b)u(b)- u(b)u’(b))- (u’(a)u(a) - u(a)u’(a))

which is an immediate consequence of the particular case p (u” and 0’’are supposed to be regulated functions in I).

=2

of (8.14.1)

(11.7.6) For any t such that a < t < b, there exists a real-valued continuous functiori x -+ K ,(x) dejiilied in I and haritig the,following properties: (a) In each ofthe intercals a < x < t , t < x < b, K , is trt)ice coritinuously diferentiable and is a solution of y” - y(x)y = 0. (b) K,satisjies the boundary conditions (11.7.2). (c) A t the point x = t, Ki(x) has a limit on the right and a limit O M the left, and K ; ( t + ) - K ; ( t - ) = -1.

352

XI

ELEMENTARY SPECTRAL THEORY

By the elementary theory of linear differential equations, there exists a solution u1 # 0 (resp u, # 0) of y” - q(x)y = 0 satisfying the condition hlul(a) f k,u’,(a) = 0 (resp 11, u2(b)+ k , u;(b) = 0), and u1 and u, are not proportional (otherwise there would be a nontrivial solution of (11.7.3) with /1. = 0 satisfying both boundary conditions (11.7.2)); hence any solution of y” - q(x)y = 0 can be written in a unique way y = clul c, u2 with constant coefficients c l , c 2 , and the function ul(x)u;(x)- u2(x)u;(x) is a constant d # 0 (by (8.14.1)). We now have only to choose the constants cl, c, such that the function K, equal to clul for a < x Q r , to c2 u, for t < x < b, should be defined and continuous at the point t , and satisfy condition (c), which yields the relations C l U 1 ( t ) - c, u,(t) = 0

+

clu’,(t)- c2 u;(r) = 1 and therefore gives c1 = -u,(t)/d, problem.

c,

=

-u,(t)/d as the solution of our

We say (by abuse of language) that K , is the elenientary solution of 3”’ - q(s)jI = 0 corresponding to the singularity t (cf. Chap. XXllI); the

function ( t , s)-+ K , ( s ) is also written K ( t , s) and called the Green function corresponding to the Sturn-Liouville problem under consideration. I t is only defined for a < r < 0, a ,< s Q b, equal to -u2(t)ul(x)/d for x d t, to - u,(t)u,(s)/dfor x 3 t , hence is continuous, and moreover can be extended by continuity for t = a and t = b by taking K(a, x) = -ul(a)u,(s)/c/ and K(b, x ) = -u2(b)u1(x)/d;i n addition it has the syninietry property (11.7.7)

K ( t , X) = K(x, t )

as follows at once from its expression.

(11.7.8) In order that a function y ( x ) be a solution o f the equation y” - q(x)y = f (x) and rwify the boundary conditions (11.7.2), it is necessary

JLr

and suficient that y ( x ) = - K ( t , x ) f ( t ) dt (f being a complex-valued regulated function in I , which is continuous except at a finite number of points of 1). (a) Sujiciency.

As

the verification of the differential equation (at the points where .f is continuous) and of the boundary conditions, reduces to routine computations of derivatives (and use of (8.7.3)).

7 THE STURM-LIOUVILLE PROBLEM

353

(b) Necessity. Apply the identity (11.7.5) in both intervals a < t < x and x < t < 6, with u ( t ) = y ( t ) and o(t) = K,(t); the relation y ( x ) = -Jab K ( t , x ) f ( t ) dt follows at once from the properties (11.7.6) of the Green function. From (1 1.7.8) it follows that any solution of the Sturm-Liouville problem is a solution of the Fredholm integral equation with hermitian kernel: (11.7.9)

where g(x) = -

c

K(t, x ) f ( t )dt,

and conversely. The incerses I , of the eigenvalues # O of the operator U in the prehilbert space G defined in (11.2.8), corresponding to the kernel function K, are called the eigencalues of the Sturm-Liouville problem. We can now state the following theorem which solves the Sturm-Liouville problem in every case: (11.7.1 0)

For any real-iwlued coritiiiuoirsjunctioii q(x) in the compact interval

[a, b]: (a) The Stirrni-Liourille probleni has an injinite strictly increasing sequence ofeigencalues (A,,) which art real iiufiibers such that lim I , = + co and that the 1

=

series

n+m

C ]/A: is convergetit.

(b) For each eigeiicalue I , , the honiogeiieous Sturm-Liouoille problem has a real valued solution cp,(x) such that cp:(x) dx = 1, and every other solutiort is a constaiit multiple of cpn . (c) The sequence (cp,) is a total orthonormal system in the prehilbert space

Lb

G (notation of Section 1I.6). (d) Let M' be a coniplex-died continuous function in I, which is the primitive of a regulatedfitnction w', such that: (i) IV' is continuous in I except at ajinite number of interior points; (ii) iv' has a continuous deriaative w" in each interid in which it is continuous; (iii) w satisjies the boundary conditions (11.7.2). Then, if c, = (wI cp,) iv(t)cp,(t) dt, we have w(x) = c,, cp,(x)

=Ib

1 n

where the series is absolutely and uniformly coiiilergent in I . (e) I f A is not one o f t h e eigenralues 2, ,for each regulated functionf, continuous in 1 except at a jinite number of points, the Sturni-Liouoille problem has a unique solirtioti 1.1' which is such that c, = (w 1 v,) is gicen bj' the formula c, = d , / ( ~- I,), wliere d,, = jubj(t)cpn(t) dt.

354

XI

ELEMENTARY SPECTRAL THEORY

(f) For A = A,,, a necessary and sujicient condition for the Sturin-Liouoille problem to have a solution is that f (t)cp,(t) dt = 0. Then, for any solution w, c,, = ( w I cp,) is arbitrary, and for m # n, c, is gizien by the same ,formula as in (e).

C

The homogeneous Sturm-Liouville problem cannot have two linearly independent solutions, otherwise it would have solutions y for which y ( a ) and y’(a) are arbitrary, which is absurd; this proves (b). The fact that all eigenvalues I,,are real follows from (11.7.7) and (11.5.7) ; moreover it follows from (1 1.7.4) that at most finitely many A,, are negative. By Mercer’s theorem ((11.6.7) and (1 1.6.8)), we have for the Green function (11.7.10.1 )

K(t,

=

1

C - qn(t)Vn(x> n An

the series being absolutely and uniformly convergent in I x I (it is supposed, as we may, that 0 is not one of the A,). We observe that (d) follows from (11.6.3) and (11.7.8) when the additional assumption is made on u’ that it!’ is continuous in 1. To prove (d) in general, let ti (1 d i d wz) be the points of f where M)’ has a discontinuity, and let cli = w’(ti+) - w ’ ( t i - ) . Then the function u = w

m

+ i=

ctiK,, satisfies all the conditions of (d) and in addition 1

has a continuous derivative, by (11.7.6). Using (1 1.7.10.1 ) we conclude the proof of (d). From the fact that the identity mapping of E = +?,-(I) into G is continuous, it follows that for the functions w satisfying the conditions of (d), we can also write MI = c,cp,, the sequence being convergent in the n

prelzilbert space G. To prove (c) it will then be enough to show that the set P of these functions lli is dense in G. Now, for any function u E G, consider the continuous function M’, equal to u in [~+.!-,b-~], m m

to a linear function x + M X + fi satisfying the first (resp. second) bounduy condition (11.7.2) in

and to a linear function in each of the intervals

7 THE STURM-LIOUVILLE PROBLEM

355

We can in addition suppose that at the points a, b, the value of w, is 0 or 1 ; it is then clear that lu(x) - w,,,(x)I < llull

+ 1 in each of the intervals

and therefore IIu - iv,,J2 is arbitrarily small by the mean value theorem; as w, satisfies all conditions in (d), this proves our assertion. Once (c) is thus proved, it is clear that the total sequence (cp,,) must be infinite, and (applying (11.6.2)), (a) is also completely proved. Finally, (e) and (f) follow at once from (11.5.11).

Remark. It is possible to obtain much more precise information on thecp,and A,,, and to prove in particular that AJn2 tends to a finite limit (see Problems 3 and 4).

PROBLEMS

1. Let 1 = [ a , b] be a compact interval in R , and let Ho be the real vector space of all real-valued continuously differentiable functions in I; Ho is made into a real prehilbert space by the scalar product (x 1 y) =Job (x’y‘

+ xy) df .

(a) Show that Ho is separable (approximate the derivative of a function x E Ho by polynomials (7.4.1)); Ho is therefore a dense subspace of a separable Hilbert space H (6.6.2).

(b) If (x.) is a Cauchy sequence in the prehilbert space Ho , show that the sequence (x.) is uniformly convergent to a continuous function u in I, and that if (y,,)is a second Cauchy sequence in Ho having the same limit in H, then (yn)converges uniformly in I to the same function v ; the elements of H can thus be identified to some continuous functions in I, which however need not be differentiable at every point of I. (Observe that for every function x for any function z z’(u) = z’(b)

=

E

E

H o , Ix(t) - x(u)l

< (t - a ) 1 / 2 ( j o b x dt)’” ’2

in I.) Show that,

Ho which is twice continuously differentiable in I and such that

0,(v I z) = -

\

uz“ dt

. a

+

fab

uz dt.

(c) Let a , ,8 be two real numbers, q a continuous function in I. Show that in N o , the function x

+ @(x) =

s.“

(x” -tqx2)df

-

~ ( x ( u )-) P(x(b))’ ~ is continuous. Let A be the

subset of H consisting of the functions x such that

Jab

x 2 dt = 1 (observe that this is not

a bounded set in the Hilbert space H). Show that in A n H o , the g.1.b. of @ ( x ) isfinite. (One need only consider the case a > 0, ,8 > 0. Assume there is a sequence (x,) in

356

XI

ELEMENTARY SPECTRAL THEORY

A n H, such that lim rD(x,) = - m, and, if y,, = n-

dt)

liZ

, lim yn = -1 cu ; consider n-m

m

the sequence of the functions y ,

= x,/y,,

and derive a contradiction from the fact that,

y', dt 0, and on the other hand, there is an interval [a,c] jab a number p > 0 such that Iy.(t)l > p for every n and every point t [a, c].)

on one hand lim

=

n-m

C

I and

E

(d) Let p1 be the g.1.b. of @(x) in A n H o . Show that if (x.) is a sequence in A n Ho such that lim @(x,) = p,,(x,) is bounded in H (same method as in (c)). Deduce from n-t m

that result that, by extracting a convenient subsequence, one may assume that the sequence (x,) is uniformly convergent in I to a function u (which, however, need not a priuri belong to H) (use Ascoli's theorem (7.5.7)). (e) @(x)is a quadratic form in Ho , i.e. one has @(x t y ) = @(x) @ ( y ) 2 'Ux, y ) , where Y is bilinear; for any function z which is twice continuously differentiable in 1

+

and such that z'(n) = z'(b) = z(a) = z(h) = 0,one has 'Y(x, z ) =

-jab

XZ"

-'

dt

+lab

qxz dt ;

z ) can be defined by the same formula for any function v continuous in 1. Show that for any such function z and any real number [, one has '€'(u,

and deduce from that result that one must have

lab+ (uz" - quz

p I u z )dt = 0.

Hence, if w is a twice continuously differentiable function such that w" = qrc - p l u , one has [ab (u - w)z" dt = 0 by integration by parts;conclude that u - w is a polynomial

of degree < 1 (observe that by substracting from u - w a suitable polynomialp of degree 1, there exists a function z such that z" = u - w - p, z(a) = z(b) = z'(a) = z'(b) = 0). Hence u is twice continuously differentiable, satisfies the differential equation us - qu

+ plu

u2 dt = 1 ; furthermore, u'(a)

last statement, express that for any z 2.

E

= 0,

= - au(a),u'(h) = pu(6).

H,, cD(u

+ [z) > p1 J

" (u a

(To prove the

+ [ z ) dt,foranyreal ~

number 6.) (a) With the notations of (11.7.10), suppose first that k l k 2 # 0, and let a = h,/k,, p = - h 2 / k 2 . Show that the q ncan be defined (up to sign) by the following conditions: (1) p, is such that, o n the sphere A : (yl y ) = 1 in G , the function (I) (defined in Problem l(c)) reaches its minimum for y = pl,and that minimum is equal to A, ; (2) for n > 1 , let A, be the intersection of A and of the hyperplanes ( y I pr)= 0 for 1 < k c IZ - 1 ; then F~is such that on A,, CD reaches its minimum for y = rp", and that minimum is equal to A,. (The characterization of y1 follows at once from the results of Problem 1; use the same kind of argument to characterize vn.) (b) If k l = 0, k2 # 0,prove similar results, replacing a by 0 in @, but replacing the sphere A by its intersection with the hyperplane in G defined by y ( a ) = 0. Proceed similarly when k , # 0 and kZ = 0, or when k , = k 2 = 0.

7 THE STURM-LIOUVILLE PROBLEM

357

(c) Under the assumptions of (a), let z , , . . . , z , - ~ be n - 1 arbitrary twice continuously differentiable functions in I, and let B(zl, . . . , z " - ~ )be the intersection of A and of the n - 1 hyperplanes ( y I z x )= 0 ( I < k < n - 1). Show that in B(zl, . . .,zn-,), the function CU reaches a minimum p(zl, .. ., z,- at a point of B(zl, . , . , zn-,),and that h, is the 1.u.b. of p ( z l , . . . , z . - ~ ) when the z , vary over the set of twice continuously differentiable functions in I (the "maximinimal" principle; same method as in (a) to prove the existence of the minimum; the inequality is proved by the same method as in Section 11.5, Problem 8). Extend the result to the cases k , k 2 = 0. 3. (a) One considers in the same interval I two linear differential equations of the second order y" - y I y hy = 0, y" - q2y hy = 0, with the same boundary conditions (11.7.2); let (hi')),(hi2))be the two strictly increasing sequences of eigenvalues of these two Sturm--Liouville problems. Show that if 41 < q 2 ,then hi') < hiz) for every - hb2)l < M for every n (use the maximinimal n,and if I q l ( t ) - q2(t)l < M in I, then principle). (b) Conclude from (a) that there is a constant c such that

+

+

for every 11, with I : h - a . (Study the Sturm-Liouville problem for the particular case in which y is a constant.) 4. (a) Let y be any solution of (11.7.3) in I = [a,h ] for h > 0. Show that there are two constants A, w such that y is a solution o f the integral equation

(*I

y ( t ) = A sin J h ( t

1.

+ w ) -1- I

dh

t

q(s)y(s)sin Jh(t

- s) ds.

Show that there exists a constant B independent of h, such that A' < B(y I y ) (use Cauchy-Schwarz in order to majorize the integral on the right-hand side of (*)). (b) Deduce from (a) that if, in the Sturm-Liouville problem, k 1 k 2# 0 or k l = k2 = 0, then there are two constants C o , C,, such that, for every n, and every t E I lp,,(t)- J2// sin Jhn t 1

< co/n

and

Ipi(t) - 4 211JXn cos JGtl

< c,

with I = b - a

(use (a), and the result of Problem 3(b)). What is the corresponding result when only one of the constants k , , k 2 is O ?

APPENDIX

ELEMENTS OF LINEAR ALGEBRA

Except for boolean algebra (Section 1.2) there is no theory more universally employed in mathematics than linear algebra; and there is hardly any theory which is more elementary, in spite of the fact that generations of professors and textbook writers have obscured its simplicity by preposterous calculations with matrices. We shall give a brief survey of the concepts and results of linear algebra which are used in this book. For a more complete account we refer the reader to the works of Halmos [I I], Jacobson [13], or Bourbaki [4].

1. VECTOR SPACES

(A.l.1) Throughout this appendix, K is a fixed (commutative) field, whose elements are called scalars. A vector space orer K (or simply a vector space, if there is no ambiguity about K) is a set E endowed with the structure defined by two mappings: (x, y ) --f

x +y

(A,x) + Ax

of E x E into E

(called addition)

of K x E into E

(called scalar multiplication)

having the following properties:

+y + z)

(1.1)

x + ( y + z ) = (x + y ) + z

(1.2)

x+y=y+x

(1.3)

there exists an element 0 E E such that x = 0

(written x

+ x for all x E E l

t Experience shows that, contrary to the opinion of some authors, there is no risk of confusion in using the same symbol 0 to denote the zero elements of all vector spaces (and of K in particular). 358

1 VECTOR SPACES

for each x E E there exists an element - x (A p)x = Ax px ~ ( xy)= + A~ %(/AX)= (Ap)x 1 * x =x

(1.4) (11.1) (11.2) (11.3) (11.4)

+ +

E

+

E such that x

+ (-x)

359

=0

(in these conditions, x, y , z are arbitrary elements of E and 2, p are arbitrary elements of K). Conditions (1.1) to (1.4) express that E is a conmutatiue group with respect to addition. It follows that, if ( x i ) i s is any ,finite family of elements of E, the sum E x i is unambiguously defined. (We recall the convention that

c

isH

xi

= 0.)

is @

+

+ +

From (1.3) and (11.2) we deduce that Ax = A(x 0) = Ax 20, and therefore A0 = 0 for all A E K . From (11.1) it follows that Ax = (A 0)x = 3.x + Ox, so that Ox = 0 for all x E E. Finally, from these two relations and (1.4), (11. I), and (11.2) we deduce that ix + A( -x) = A0 = 0, and Ax + (-A)x = Ox = 0, so that (- J.)x = A( - x) = -(Ax). The elements of E are often called vectors. (A.1.2) An additive group consisting of the element 0 alone is a vector space: the scalar multiplication is the unique mapping of K x ( 0 ) into ( 0 ) . The field K is a vector space over itself, the scalar multiplication being multiplication in K. If K’ is a subjield of K and if E is a vector space over K, then E is also a vector space over K’ if we define the scalar multiplication to be the restriction to K’ x E of the mapping (A, x) + Ax. (A.1.3) If E is a vector space, a vector subspace (or simply subspace) of E is defined to be any subset F of E such that the relations x E F, y E F imply Ax -t~ c Ey F, for all scalars A, p . The restriction to F x F (resp. K x F) of the addition (resp. scalar multiplication) in E is a mapping into F, and therefore F a vector space with respect to these mappings. This justifies the terminology. If (xi)l ;,<,, is any finite family of vectors in F, and if O.i)lsisnis a family ,

of scalars, then by induction on n we see that form

n

i= 1

3Lixi(where xi E E and

n

1AixiE F. A vector of the

i= 1

A; E K) is called a linear combination of the

x i . (A linear combination is always aJinite sum, even if E is, for example, a normed space (Section 5.1) in which certain infinite sums are defined (Section

5.3))

360

APPENDIX:

ELEMENTS OF LINEAR ALGEBRA

In any vector space E the subsets ( 0 ) and E are vector subspaces (called the tricial subspaces). If (M,),e, is any family of vector subspaces of E, the intersection M, is again a vector subspace. If A is any subset of E, the

n

as1

intersection M of the subspaces which contain A (such subspaces exist, for example E itself) is therefore the smallest subspace containing A. We say that M is generated by A, or that A is a systeni of generators of M. (A.1.4) The i m i o r subspace M generated b y A is the set of all linear conibitiations o j j n i t e families of elenietits of A.

It follows from above that such linear combinations belong to every vector subspace containing A, and therefore belong to M. Conversely, if x = & l x r and y = d J y j are two linear combinations of elements of A, i

then so is 1,x

+ py = i

+ 1(pdj)yJ. Hence the set of all such linear

J

(l.yl)xi

J

combinations is a vector subspace of E, which contains A (by (11.4)) and therefore coincides with M .

In particular, take A to be the unioti of a family (NP)PeJof vector subspaces of E. Every linear combination of elements of A i s then of the form x, x, . . . xP,, where (fl,), jdn, is any finite family of elements of J, and xPJE N P Jfor each j . The set of all these sums is therefore the smallest vector subspace containing all the N,. It is called the sum of the N, (not to be confused with their union!) and is denoted by N, . When J = { I , 2},

+

+

+

+

1

P E J

the notation N, N, is also used, and similarly for any finite set of indices, It is clear that M N = N M and M ( N + P) = (M + N ) + P for any three subspaces M, N,Pi n E. The relation M c N is equivalent to M + N = N. For every x E E, the subspace of E generated by x is written Kx (and is sometimes called a " ray '' when x # 0). If (x,), A is any family of vectors in E, the subspace generated by the family is therefore Kx,,

+

+

+

asA

(A.1.5) Let ( E J U E lbe a/iy family of vector spaces. We shall construct a new vector space E, called the direct sun? (or external direct sum) of the family (E,) and denoted by @ E,. The elements of E are the families ael

x = (x,), E , , where x, E E, for all ct and x, = 0 f o r all except a j n i t e nuniber of indices. The vector x u is called the coniponent of index M in x. Addition and

scalar multiplication in E are defined by the formulas

2 LINEAR MAPPINGS

361

((Ax,) belongs to E because 10 = 0). The conditions of (A.l .I) are immediately verified (the element 0 of E is the family (x,) for which x, = 0 for all e). When J = (1, 2}, the notation El 0E, is used, and similarly for any finite set of indices. When J is finite, the set E is equal to the product set E,.

fl

aeJ

If J is arbitrary and H is a subset of J (other than @ or J), the vector space @ E, can be identified in an obvious way with

ael

@ E, (as"

0 @ En). (aeJ-H

When all the E, are equal to K, their direct sum is denoted by K'". It is the set of all mappings e -+ ).(A of I into K such that A(e) = 0 for all but a finite number of indices e E I.

2. L I N E A R MAPPINGS

(A.2.1) Let E, F be two vector spaces over the same field K. A mapping u : E F is said to be linear if it satisfies the condition --f

(A.2.1 .I)

u(Ax + P A = Au(x) + P ( Y >

for all scalars A, p and all vectors x, y in E. In particular, we have u(0) = 0. By induction on n it follows from (A.2.1 .I) that (A.2.1.2) i Q n is a family where (xi)l is any finite family of vectors in E, and of scalars. A linear mapping of E into E is called an endomorphism of E. A linear mapping of E into K is called a linear form on E.

(A.2.2) Let u : E -+ F be a linear mapping. If M is any subspace of E, it is immediately verified that its image u(M) is a subspace of F. If N is any subspace of F, then its inverse image u-'(N) is a subspace of E. If ( M a ) is any

family of subspaces of E, then u ( T Ma)=

a

u(M,).

In particular, u(E) (which is called the image of u and is written im(u)) is a subspace of F, and u-'(O) (which is called the kernel of u and written ker(u)) is a subspace of E. The mapping u is injective if and only if u-'(O) = {O}, because the relation u(x) = u(x') is equivalent to u(x - x') = 0. If u is bijective, it is called an ison?orphism of E onto F. (If also F = E, then u is an autoniorphism of E). If u is bijective, it is clear that the inverse mapping u-' : F -+ E is linear, and therefore an isomorphism of F onto E.

362

APPENDIX: ELEMENTS OF LINEAR ALGEBRA

Two vector spaces E, F are said to be isomorphic if there exists an isomorphism of E onto F. In that case, any theorem proved for E, involving vectors and subspaces of E, immediately gives a corresponding theorem for F, involving the images of the vectors and subspaces in question. If u : E -,F is an injective linear mapping, then u can be considered as an isomorphism of E onto its image u(E). Examples of linear mappings (A.2.3) The identity mapping of a vector space E onto itself (often denoted by 1 or IE, or simply 1 or I ) is linear. So is the unique mapping of E into a vector space consisting of 0 alone. For every I E K, the mapping h, : x -+ Ax is an endoinorphism of E, called the homothetic mapping with ratio I . If I = 0, its image is the zero subspace ( 0 ) of E. If I # 0, then hl is bijective and its inverse automorphism is the homothetic mapping with ratio A- because we have x = I - ’ ( I x ) by (11.3). For each x, E E, the mapping 5 -+
’,

aEI

I we define a linear mapping j , : E, -+ E by the rule where yp = 0 if # a, and J’, = x, . It is clear that j , is j,(x,) = injective, and hence its image j,(E,) is a subspace of E, isomorphic to E,. This subspace is called the component subspace of index a in E, and is often identified with E,. We also define linear mappings p a : E -+ E, (for each M E I) as follows: if x = (xp)pe I is an element of E, then p,(x) = x u , It is clear that p , is surjective. If I is finite, p a is the projection with index CY (Section 1.3). For each x E E, the set H of indices M EI such that p,(x) # 0 is finite, and we have x = j,(p,(x)). sum. For each

M E

2

asH

(A.2.4) If u : E -+ F and u’ : E F are linear mappings, and if A is a scalar, it is immediately verified that the mappings

x -+ u ( x ) + u’(x) and x -+ Au(x)

3 DIRECT SUMS OF SUBSPACES

363

are linear mappings of E into F. They are denoted by u + u’ and Au, respectively. It is then routine to check that the set of linear mappings of E into F (which is denoted by Hom,(E, F), or simply by Hom(E, F)) is a vector space with respect to the addition (u, u’) -+ u + u’ and the scalar multiplication (A, u) -+ AM. In other words, the eight conditions of (A.l . I ) are satisfied. Let u : E --t F and u : F G be linear mappings. Then their conipositiorz v o u : E -+ G is linear. Furthermore, if u’ : E -+ F and v’ : F -+ G are linear mappings and A is a scalar, then we have -+

(A.2.4.1)

2‘0

(A.2.4.2)

(0

( u + u’) = v u + v u’, 0

+ 0’)

0

u = v o u + v ’ o u,

v (AM)

(A.2.4.3)

0

0

= (Av) 0

u

= A(v

0

u).

(The verifications involved are trivial if one bears in mind the definition of equality of two mappings (Section 1.4).) The set Hom(E, E) of endomorphisms of E is also denoted by End(E) or End,@). This set, endowed with the addition (u, u) + u + v and the “multiplication ” (u, u ) -+ u 21, is a ring (in general not commutative) whose identity element is 1,. This follows from the formulas above and from Section 1.7. The property (A.2.4.3), which relates the ring structure of End(E) to its vector space structure, may be expressed by saying that End(E) is an algebra over K. 0

3. DIRECT S U M S O F SUBSPACES

(A.3.1) Let (Ea),G,be afainily of vector spaces, let F be a vector space, and for each ct E I let u, : E, -+ F be a linear mapping. Then there exists a unique linear mapping u of the direct sun? E = @ E, into F such that u j , = u, 0

for all ct E I (vit’iere j , : E,

as1

-+

E is the canonical injection deJined in (A.2.3)).

For each x E E we have x

j,(p,(x)), where H is any finite subset of

= ~

E

H

I containing all ct E I such that p,(x) # 0. Hence we must have u(x) = u(j,(p,(x))) = 1u,(p,(x)), from which the uniqueness of u follows.

c

aeH

acN

Conversely, if we de&e u by this formula, it is immediately verified that u is linear (by using the fact that, given x and y in E, we may take H to be a finite set containing all the indices ct E I for which either p,(x) # 0 or p,(y) # 0). This completes the proof.

APPENDIX: ELEMENTS OF LINEAR ALGEBRA

364

(A.3.2) Now let E be a vector space, let ( M a ) a G I be a family of subspaces of E, and for each a E I let f a : M a + E be the canonical injection. By (A.3.1) there exists a linear mapping f of M = @ M a into E such that

f o j, =fa for all

as1

c1 E

1. The image o f f is the subspace M’

Iff is injective, the sum

M, of E.

= a

~

l

1M aof subspaces of E is said to be direct. This

a .. c - 1.

therefore means that every vector x belonging to

c Mu can be expressed

c x,, where H is a finite subset of I, and x, # 0 for all a H. (If x 0 we have to take H @.) We shall usually identify c M a and @ M aby means off. as1

uniqueiy in the form x,

asH

=

E

E

E, and

=

as1

as1

c

(A.3.3) The sum of a family (Ma),€ I of subspaces of a vector space is direct ifand only $Mu n M, = ( 0 )for all c1 E I. ,+a

For to say that the mapping f defined in (A.3.2) is not injective means that there exists a finite number of nonzero elements xuiE Mai (1 < i < n) such that E x a i= 0 (A.2.2). This equation can be written in the form x,, =

c

i

( - x u , ) , and expresses that x, belongs to M a , n

j> 1

c M, . Hence the

B+ai

result.

(A.3.4) If E is equal to the direct sum of two subspaces M and N, we say that M and N are supplementary subspaces of E (or that M (resp. N) is a supplemenl to N (resp. M)). This signifies that M + N = E and M n N = ( 0 ) . Hence we have linear mappingsp : E -+ M and q : E -+ N, the “projections” of E onto M and N, such that x = p(x) q(x) for all x E E. The kernel o f p (resp. 4 ) is N (resp. M), and P(P(XN = P W , 4 ( 4 W = 4(4. We cannot speak of “ t he” supplementary subspace of a subspace M, because in general there is more than one. However, there is the following result :

+

(A.3.5) Let M, N, N‘be subspaces of E, such that M and N are supplementary, and M and N‘are supplementary. Let q : E N be the projection of E onto N corresponding to the direct sum decomposition E = M @ N. Then the restriction q’ : x + q(x) of q to N‘ is an isomorphism of N’ onto N. -+

4 BASES. DIMENSION AND CODIMENSION

365

For since the kernel of q is M, the kernel of q’ is M n N’ = ( 0 ) and therefore q’ is injective. On the other hand, if x is any element of N, there exist y E M and z E N’ such that x = y + z, so that q(z) = q(x) = x, because q(y) = 0. Hence q’ is surjective.

4. BASES. D I M E N S I O N A N D C O D I M E N S I O N

(A.4.1) A family ( x , ) , ~ , of vectors in a vector space E is said to be .free, and the x, are said to be linearly independent, if for each finite subset H of I the relation A,x, = 0 implies that A, = 0 for all a E H. It follows that n c H

x, # 0 for al7-i E I, and xp # x u if fi # a (otherwise we should have 1 . xu ( - l)xp = 0). The subspaces Kx, are therefore “rays” isomorphic to K , and the linear independence of the x, may be expressed by saying that the x, are all nonzero and the sum of the rays K x , is direct.

+

(A.4.2) The family (xu),€ I is free fi and only ih for each a E T, the vector x, does not belong to the subspace generated by the xp with fi # a.

This follows from the remark above and (A.3.3). (A.4.3) Let u : E -+ F be a linear mapping, and let (y,), be a free family of elements of u(E). For each a E I, let x, E E be such that u(x,) = y,. Then the Kx, is direct. family ( x , ) , ~I is.free, and the sun1 of ker(u) and

1

as1

Suppose that, for some finite subset H of I, we have a relation of the form

1A, x, + y = 0, where y E ker(u). It follows that 1A, u(x,) = 0, because aeH

acH

u(y) = 0. Since the family ( ~ ( x , ) ) , is ~ ~free, we deduce that A, and hence that y = 0. Q.E.D.

=0

for all a,

(A.4.4) A family (b,), I of vectors in E is said to be a basis of E if it is free and generates E; or, equivalently, if the sum Kb, is direct and equal to E;

1

, E l

or, equivalently again, if for each x E E there exists a unique finite family of such that H c I and Am # 0 for all a E H, and x = A, b, . scalars (A,), aoH

Every free family (xu),€, is therefore a basis of the vector subspace

366

APPENDIX: ELEMENTS OF LINEAR ALGEBRA

In the vector space K('), we denote by e, the vector (6,,),,, such that 6,, = 0 if p # a, and 6,, = 1. Clearly the family (ea)aeis a basis of K('). This basis is called the canonical basis of K('). If a vector space E has a basis (b,), ,I , then there exists a unique isomorphism of K(') onto E which maps emto b, for each c( E I (A.3.1).

,

(A.4.5) Let V be a vector subspace of a vector space E, and suppose that E is generated by the union of V and the set of vectors of a j n i t e family (xi)l is,,.

which is a basis of a subspace sup-

Then there exists a subfamily plementary to V in E.

The integer r is defined to be the largest for which there exists a subfamily ( x i k ) l Q k sof r such that the sum E' of V and the Kxik is direct. It is enough to show that E' = E. Suppose that E' # E. Then there is an index j such that 1 d j d n and x j $ E' (otherwise E' would contain V and all the x i , and therefore by hypothesis would be equal to E). Hence we shall arrive at a contradiction if we can show that the sum of V, the K x i kand K x j is direct. So consider a relation of the form Axj p k x i k + y = 0, with y E V. First of all, this implies that

+c k

1 = 0 (otherwise x j = definition of (

-1(A-'pk)xik k

x

~ shows ~ that )

- A-'y, which belongs to E'). Then the

p k~ = 0~for

all ~ k, ~and y~

= 0.

Q.E.D.

(A.4.6) Every vector space generated by a finite set of vectors has a basis consisting of vectors belonging to this set.

Take V

=

( 0 ) in A.4.5.

These last two results remain valid without the finiteness conditions (see [4]), but we shall not need to use this generalization in this book.

(A.4.7) Let E be a vector space with ajinite basis (bi), of E has exactly n elements.

iGn.

Then every basis

It is enough to show that every other basis of E has at most n elements, because we can then interchange the roles of the two bases. We proceed by induction on 11, the result being trivial when n = 0. Let (bi),6 , be a basis of E, and consider an element bi of this basis and the ray V = Kb; of E. The subspace V' = C Kb; is supplementary to V in E. On the other hand, afv

4 BASES. DIMENSION AND CODIMENSION

367

E is generated by V and the b i , hence there is a subfamily ( b i k ) 1 6 k Qofr ( b i ) 1 6 i Qsuch n that the i, are all distinct and such that V" = Kb,, is supple-

1 k

mentary to V in E (A.4.5). We cannot have r = n, otherwise we should have V " = E, which is absurd. Hence V" has a basis of r < n - 1 elements, and since V' is isomorphic t o V" (A.3.5), it follows that V' has a basis of r < n - 1 elements. Hence the inductive hypothesis shows that I - {y} has at most n - 1 elements, and so I has at most n elements. Q.E.D. A vector space E which has a finite basis is said to bejinite dimensional. The integer n in (A.4.7), that is to say the number of elements in any basis of E, is called the dimension of E (over K) and is denoted by dim E or dim, E. If dim E = n, then E is isomorphic to K". A vector space which is not of finite dimension is said to be injinite dimensional. The relation dim E = 0 is equivalent to E = (0). (A.4.8) (i) In a vector space E of jinite dimension n, eaery system of generators contains a basis of E and has at least n elements. Any system of generators consisting of n elements is a basis. (ii) In a vector space E of finite dimension n, every free system of vectors is contained in a basis of E and has at most n elements. Any free system consisting of n elements is a basis.

If (xi)l6 i 6 m is a free system, it follows from (A.4.5) applied to V

m

= i= 1

Kxi

jan of E that there exists a subfamily ( b j J l<,,<,. such that and a basis ( b j ) l Q the x i and the 6, form a basis of E. This proves (ii). The assertion (i) follows from (A.4.7) and (A.4.6) in the case of a finite system of generators. In the general case, it is enough to show that a system S of generators contains a free system of n elements. Suppose this is not the case, and that the greatest number of elements in a free system extracted from S is m < n. If ( Y , ) ~

is such a free system, then V =

m

K y , cannot be equal to E, by (A.4.7), and

i= 1

hence there exists z E S not contained in V. Then the y i and z form a basis of V + K z by virtue of (A.4.5), and hence form a free family of rn + 1 elements. This contradiction completes the proof. (A.4.9) A subspace V of a vector space E is said to be offinite codinzension if V has a supplementary subspace in E which is finite dimensional. The dimension in question does not depend on the choice of the supplementary subspace (A.3.5), and is called the codimension of V in E. It is denoted by

368

APPENDIX: ELEMENTS OF LINEAR ALGEBRA

codim V or codim, V. If V has no supplementary subspace of finite dimension, then V is said to be of infinite codimension in E. By (A.4.5), V is of finite codimension in E if and only if E is generated by the union of V and a finite set of vectors. (A.4.10) If E is a jinite-dinjensional vector space, every subspace F of E is jinite diirnensional and ofjinite codinwnsion in E, and

dim F + codim F = dim E.

(A.4.10.1)

For by applying (A.4.5) to F and a basis of E, it follows that there exists a supplementary subspace F’ of F in E, with dim F’ Q dim E. Interchanging the roles of F and F’, and using (A.3.5), it follows that F is also finite dimensional. If ( x i ) lC i Q p is a basis of F and if ( x ; )Q~j 4 q is a basis of F’, it is clear that the x i and the x i together form a basis of E.

(A.4.11) Let E be aJinite-dimensional vector space and let F be a subspace of E. If dim F = dim E, then F = E.

This follows immediately from (A.4.10.1) (A.4.12) Let M and N be twojinite-dimensionalsubspaces of a vector space E. Then M N isjinite dimensional, and we have

+

dim(M

(A.4.12.1)

+ N) + dim(M n N ) = dim M + dim N.

The set consisting of the elements of a basis of M and a basis of N will generate M N, which is therefore of finite dimension (A.4.6). Let P (resp. Q) be a supplementary subspace of M n N in M (resp. N). It is clear that M + N is the direct sum of M n N, P, and Q (A.3.3), and therefore dim(M N) = dim(M n N ) + dim P dim Q. But also (A.4.10) we have dim P = dim M - dim(M n N), and dim Q = dim N - dim(M n N). Hence the result.

+

+

+

(A.4.13) Let M and N be two vector subspaces ofjinite codimension in a t w t o r space E. Then M n N is ofjinite codimension in E, and we have (A.4.13.1)

codim(M

+ N ) + codim(M n N) = codim M + codim N.

4 BASES. DIMENSION AND CODIMENSION

369

Let V be a supplementary subspace of M in E. Then V is finite dimensional. Let p : E + V be the projection of E onto V with kernel M (A.3.4). The subspace p ( N ) is finite dimensional (A.4.10). Let (bJlSis, be a basis of p ( N ) , and for each i let ci E N be such that p(ci) = bi. Then N is generated by M n N and the ci (A.4.3), hence M n N is of finite codimension in N and therefore of finite codimension in E. Let P (resp. Q) be a supplementary subspace of M n N in M (resp. N ) , and let R be a supplementary subspace of M + N in E. Then E is the direct sum of M n N, P, Q, and R, so that we have codim(M n N) = dim P + dim Q + dim R, codim M = dim Q + dim R, codim N = dim P dim R, and codim(M + N) = dim R. The result (A.4.13.1) follows immediately from these formulas.

+

(A.4.14) A subspace of codimension 1 in a vector space E is called a hyperplane in E. If E is of finite dimension n, every hyperplane in E is of dimension n - 1 (A.4.10.1).

(A.4.15) I f H is a hyperplane in E, there exists a linear form f # 0 on E such t h a t f - ' ( 0 ) = H. I f f ' is another linearform on E such that f'-'(O) = H, then there exists a scalar y # 0 such that f ' = yJ Conversely, if g is any nonzero linear form on E, then g-'(0) is a hyperplane in E.

Tf H is a hyperplane, there exists a vector a 4 H such that E is the direct sum of H and Ka, and every x E E can therefore be expressed uniquely in the form x = y +f(x)a, where f ( x ) E K. Since x + f ( x ) a is linear (A.3.4), so is x - t f ( x ) (A.2.3); hence f is a linear form, and H is its kernel (A.3.4). If f ' is any linear form such that f'-'(O) = H, and if we put f ( a ) = c1 and f ' ( a ) = j, then we have c( # 0 and p # 0, and orf' - /If is a linear form on E which vanishes on H and at a, and therefore vanishes identically on E = H + Ka. This shows that f ' = 6 ' j J Finally, if H' = g-'(0), then there exists a vector b H', because g # 0. Let y = g(b) # 0. Then for every x E E we have g(x - y-'g(x)b) = 0, so that x = y + y-'g(x)b for some y E H'. This shows that E = K b H', and the sum is direct because b H'. Hence H' is a hyperplane.

+

(A.4.16) Let u : E -+ F be a linear mapping. We say that u is of finite rank if u(E) is of finite dimension. The dimension of u(E) is then called the rank of u, and is written rank(u). If u(E) is infinite dimensional, then u is said to be of injinite rank.

370

APPENDIX: ELEMENTS OF LINEAR ALGEBRA

(14.4.17) The mapping u is of -finite rank $ and only if ker(u) is of finite

codimension, and then

rank(u) = codim,(ker(u)).

(A.4.17.1)

If ker(u) has a supplementary subspace V of finite dimension, then the restriction of M to V is an isomorphism of V onto u(E), so that u(E) has finite dimension equal to dim V. Conversely, if ( b i ) 1 6 i Gisna finite basis of u(E), let aibe a vector in E such that u(a,) = bi (1 < i < n). Then E is the direct sum of ker(u) and the Kai (A.4.3). (A.4.18) Let E, F be vector spaces and let u : E -+ F be a linear mapping.

(i) /f F isfinite dimensional, then rank(u) i f and only if u is surjective. (ii) / f E isfinite dimensional, then rank(u) i f and only if u is injective.

< dim F, and rank(u) = dim F < dim E, and rank(u) = dim E

The first assertion is an immediate consequence of the definition of rank(u) and (A.4.11). To prove (ii) it is enough to observe that if dim E = n, then u-’(O) is of dimension n - rank(u), by (A.4.17) and (A.4.10). (A.4.19) Let E be a jinite-dimensional vector space and let u be an endomorphism of E. Then the following assertions are equivalent: (i) u is bijective; (ii) u is injective; (iii) u is surjective; (iv) rank(u) = dim E.

This follows immediately from (A.4.18). Let E be a vector space over a field K, and let K’ be a subfield of K. Let (bU),E , be a basis of E over K , and let (pa)n be a basis of K considered as a vector space over K’. Then the family (pl b J , where (A, a) runs through J x I, is a basis of E over K . For it is clear that E is generated (over K’) by the elements of this family. On the other hand, suppose we have t r a p nb, = 0 (A.4.20)

,

c(cSaapi)ba La

with scalars tnaE K’. This relation may be writtenintheform

a

0. Since the b, are linearly independent over K, it follows that

for each index

c(

E I,

=

1

1

Snap,. = 0

and then that ta,= 0 for each (A, a) E J x I because the

5

MATRICES

371

p A are linearly independent over K'. Hence the pn b, are linearly independent

over K', and our assertion is proved. In particular:

IfE isfinite dimensional over K, and K isfinite dimensional over K', then E isfinite dimensional over K' and we have

(A.4.21)

(A.4.21 . I )

dim,, E = dim, E * dim,. K.

5. MATRICES

(A.5.1) Let E, F be vector spaces over a field K, and suppose that E is of finite dimension n. Let ( a J I S i S be n a basis of E, so that E is the direct sum of the Ka, . By (A.3.1) there is a one-one correspondence between the linear , n vectors of F; this mappings u of E into F and the families ( b J I S i S ,of correspondence is defined by bi = #(ai)(1 < i < n). Thus the vector space Hom(E, F) is isomorphic to F".

Suppose furthermore that F is of finite dimension m, and let (bj)lgjgm be a basis of F. Then there is a one-one correspondence between the vectors y E F and the families ( r ] j ) l C j S m of m elements of K, defined by (A.5.2)

,.. m

y =

j = I

r i b j . Hence there is a one-one correspondence between the linear

mappings u : E -+ F and the " double "families ( a j i )(with 1
(A.5.2.1)

u(ai)=

C1 a j i b j

j=

(1

< i < n).

Such families are called matrices with m rows and n columns (or m x n matrices) over K. They form a vector space over K, isomorphic to K"". The subfamily (aji)laiSnis the jth row, and the subfamily ( a j i ) l s j S mis the ith column, of the matrix ( L X ~ ,G)j ~S m , The matrix M(u) = (aji)defined by (A.5.2.1) is called the matrix ofu with respect to the bases (a,) and (bj).If u, u' are two linear mappings of E into F, and if A is any scalar, then M(u + u') = M(u)

+ M(u'),

M ( l u ) = AM(u), the matrices being taken in each case with respect to the same bases (a,) of E and ( b j ) of F.

372

APPENDIX: ELEMENTS OF LINEAR ALGEBRA

(A.5.3) Now let G be another finite-dimensional vector space over K, and k ~ap basis of G. Let u : E-+ F and u : F-+ G be two linear let ( c h ) ~ ~ be mappings and let w = u u : E --t G. Suppose that the matrix M(u) of u with respect to ( a , ) and (bj),and the matrix M(v) of u with respect to (bj) and (ck) are known, and let us calculate the matrix M(w) of w with respect to (a,) and (ck). If M(u) = (aji), M(v) = (/Ihj), M(w) = ( y k i ) , then by definition we have 0

from which it follows that m

(A.5.3.1)

Yki

=

C B h j aji

j =1

*

Thep x n matrix M(w) is said to be the product of thep x m matrix M(u) and the m x n matrix M(u), and we write M(u 0

24)

= M(v)M(u).

Thus the product of two matrices is calculated by “ multiplying the rows of the first by the columns of the second.” Having given these definitions, it is of course possible to translate into matrix language most of the results we have established for linear mappings. We shall not stop to do this: indeed, in practice it is almost always advantageous, when presented with a problem of matrix algebra, to reformulate it in the language, so much more flexible and appropriate, of linear mappings. For example, there is no simple interpretation, in terms of matrices, of the essential concepts of the kernel and the image of a linear mapping.

6. M U L T I L I N E A R MAPPINGS. D E T E R M I N A N T S

Let El, . . . , E, and F be r + 1 vector spaces over a field K . A mapping u : El x E, x . . . x E, -+ F is said to be r-linear if it is “ linear in each of its arguments”: that is to say, for each i = 1,2, . . . ,r and each choice of elements aj E Ej ( j # i ) , the partial mapping (A.6.1)

xi -+ u ( q , . . . , ai-1, x i , ai+1, . . . ,a,) of Ei into F is linear. This implies in particular that

u(a,, *

. . ,Ui-1,0, U i f l , . . . ,a,) = 0

MULTILINEAR MAPPINGS. DETERMINANTS

6

373

for all choices of the a j . By induction on n, it follows from the definition that

where the sum on the right-hand side is over all systems ( j l , . . . ,j,) such that 1 < j , < n, . . . , 1 < j , < n; the t i jare scalars and the x i j are elements of E, (1 < i < r , I < j < n ) . An r-linear mapping of E, x * * . x E, into K is called an r-linear form. (A.6.2) Suppose that each of the vector spaces Ei (1 < i < r) has afinite basis (b,j)l j d n , . Then it follows from the formula (A.6.1 .I) that an r-linear mapping u of El x E, x * . . x Er into F is uniquely determined by the vectors u(blj , , b Z j , ., . . , b,,,) E F ( I < ji< n, for each i = 1 , 2, . . . , r). Conversely, if we are given any system ( c j ,j , j p ) of n,n2 . . n, vectors in F, then there exists a unique r-linear mapping u of El x E, x - . . x E, into F such that u(b,j , , b 2 j , , . . . , brjr)= c j lj , ,..j , for each system of indices ( j , ,j , , . . . , j,). To see this it is enough t o define u by the formula (A.6.1 . I ) (taking n there to be greater than all the n i , with x i j = b,, for j < n , and x i j = 0 for j > n,). It is immediately verified that the mapping u so defined is indeed r-linear. _,,

9

(A.6.3) Consider the case where all the E,areequal tothesamevectorspaceE. An r-linear mapping u : E' -+ F is said to be alternating if u ( x l , x , , . . . , x,) = 0 whenever there are two distinct indices i < j such that x i = x j . It follows from this definition that

+ x j , Xi+,, . , x j - I , xi + . . . , x,) * ., xi,. . . , . . . , x,) + u(x,, . . . , . . . , xi,. . . ,x,)

0 = u(x,, . . . , xi-1, xi = u(x,,

I

xj, X j f l ,

* *

xj,

xj,

for all x i E E. ln other words, if we interchange two of the arguments x i , x j in u, the value of u changes sign. Since every permutation 0 of the set { I , 2, . . ., r} may be expressed as a product of transpositions, the following formula is an easy consequence: (A.6.3.1)

u(xu(l),xu(2) . * * 9

9

xo(r))

= ~u ~(x,, x2 2

. . > xr), *

where E, is the signature of the permutation 6. If E is finite dimensional and ( b j ) l s j Q nis a basis of E, then the u(bj,,bj, , . . . , b j r )are zero by definition whenever two of the indices j,, . . . ,j , are equal. By virtue of (A.6.3.1), the values of u(bjl,. . . , bj,) for sequences of distinct indices ( j , ) are determined by those which correspond to increasing

APPENDIX: ELEMENTS OF LINEAR ALGEBRA

374

--

sequences j , < j , < < j , . Conversely, if we assign an arbitrary element c ~ , ~ ~ of , . .F , ~ to each increasing sequence, then there exists a unique alternating r-linear mapping u : E' -,F such that u(bj,,b,, , . . . , bjp) = c j l j ,.,,j , whenever j , < j , < . . . < j , . The verification of this statement is left to the reader. (A.6.4) Consider the particular case of alternating n-linear forms on E", where n is the dimension of E. The remarks above show that such a formf is completely determined by its value f ( b , , b, , . . . , b,,), where (bi) is any basis of E; andfis identically zero if and only iff(b,, . . . , b,) = 0. It follows that, if f o is one of these nonzero forms, then every other alternating ti-linear forms on E" can be written as A fo , uhere A is a scalar. These hypotheses and notations will remain in force for the rest of this section.

Let u be an endomorphism of E. Then there exists a unique scalar det(u) such that, i f f is any alternating n-linear form on En, we haue: (A.6.5)

(A.6.5.1)

f ( u ( x , ) , u(x,), . * * 4 x n ) ) 5

for all choices of x i E E ( 1

= det(u)

.S(X,,x2 . 9

9

*

9

xn)

< i < n).

It is enough to prove this for,fo, in which case the result follows from . . ., u(x,)) is an alternating n-linear form on E". (A.6.4) and the fact that ( x l , .. ., xn)-,fo(u(xl),

The scalar det(u) is called the determinant of

Clearly we have

det( lE) = 1.

(A.6.5.2) (A.6.6)

11.

r f u , u are two endomorphism of E, then det(u u) = det(u) det(u).

(A.6.6.1)

By applying (A.6.5) to the alternating n-linear form (x,,

. . . ,X") +fO(U(Xl),

'

. ,U(X">)

and the endomorphism u, we obtain ~ O ( U ( ~ X I* .) *) , ~(~(xn))) = det(u)f0(4~1),* * * 3

= det(v) det(u)fo(x,,

9

u(Xn>>

. . . ,xJ.

Since fo is not zero, the formula (A.6.6.1) now follows from the definition of det(u t i ) . 0

MULTILINEAR MAPPINGS. DETERMINANTS

6

(A.6.7)

375

det(u) # 0 ifand only if u is bijective.

If u is bijective, it has an inverse u-l such that zi u-' = 1., Hence, by (A.6.6) and (A.6.5.2), we have det(u) det(i4-') = I , so that certainly det(u) # 0. If u is not bijective, then it is not injective (A.4.19), hence there exists b, # 0 such that u(b,) = 0. There exists a basis (b;), of E containing 6 , (A.4.5), and we have fo(b,, . . . , 6,) # 0, whereas fo(u(b,), . . . , u(b,)) = 0. Hence det(u) = 0. (A.6.8) Let ( b , ) l s be a basis of E, and let M ( u ) = ( z j i ) be the matrix of u with respect to the two bases (b;)and (b;)of E (or, as is usually said, tllenzatrix of I I witA respect to the basis (b;)). Since ,fo(h,, . . . , b,) # 0, the formulas (A.6.1 .I)and (A.6.5.1) give (A.6.8.1)

det(4

=

c

EU

%(l)lMU(2)2

..

*

%("),

3

U

where CJ runs through the symmetric group 6,of all permutations of (1, 2, ..., n}. The determinant of the matrix M(u) is by definition the determinant of u. This provides the link between our theory and the classical theory of determinants in its original form. We shall not need to use this latter theory, and we leave the task of transcribing our results into the old-fashioned notation to those readers who are interested in this type of calculation. In applications it is always much simpler to go back to the definition (A.6.5), as we shall illustrate by considering the eigenvalues of an endomorphism. (A.6.9) The definition of the eigenvalues of an endomorphism u is that given in ( l l . l . l ) , except that the field C is now replaced by an arbitrary field K. It follows immediately from (A.6.7) that these eigenvalues are the roots of the equation (called the characteristic equation of u)

det(u - 1 lE) = 0.

(A.6.9.1)

The formula (A.6.8.1) shows immediately that the left-hand side of this equation is a polynomial of degree n in A, with leading coefficient (- 1)". In what follows we shall assume that the field K is algebraically closed, so that det(u - A IE) factorizes into linear factors (A, - A)(& - A) * . (A, - A). (A.6.10) (A.6.10.1)

There exists a basis (b,, . . . , 6,) of E such that

+

u(bJ = A j b j ai.;+,bi+,

+ + ainh, 1 . -

(I

< i < n).

376

APPENDIX: ELEMENTS OF LINEAR ALGEBRA

Conuersely, i f (bi)l

is a basis with this property, then

det(u - A lE) = (A, - A)(A2 - A)

* . . (A, - A).

The proof is by induction on n. By hypothesis, there exists a vector b, # 0 in E which is an eigenvector for the eigenvalue A,; in other words, u(b,) = A, b, . Let us split E into a direct sum Kb, V, and let p : E + V be the corresponding projection (A.3.4). The mapping x -p(u(x)) is an endomorphism of V, and hence there exists a basis (bl, . . . , bnwl)of V such that

+

P(u(bi))=pibi

+ Ui,i+lbi+l +

+ ~ ( ~ , , - ~ b , - , (1 < i < n - 1)

and consequently

u(bJ

= pibi

+~

+ +

( ~ , ~ + ~ bai,n-.lb,,-l ~ + ~

+ ai,,b,

(1

< i < n - 1)

for suitable scalars m i , . We now have

fo(~(b1) - Ab1, . . ., u(bn) - Abn) =fo((/ll

- A)bi

* * . > (Pn-1

+ . + alnb, , *

-1)bn-i

A)b2

( ~ 2

+. + * *

~ 2 b, ,

,.. .,

+an-1,nbn,(An-A)btJ.

If we expand the right-hand side by means of (A.6.1 . I ) and use the definition of an alternating multilinear form, we see easily that the only term which does not vanish is (PI - m

2

- A)

. . . (Pn- 1 -

A W L ,

-

A)fO(b,, . . . , b,),

(P,,-~ - A)(A, - A). This and therefore det(u - A . lE) = ( p l - A)(p2 - A) proves that the p i are (except possibly for their order) the scalars A,, . . . , An-l, and the calculation above also establishes the second assertion of (A.6.10). The matrix of u with respect to a basis satisfying the conditions of (A.6.10) is said to be lower triangular.

(A.6.11) (A.6.11 . l )

For each integer k > 0, we have det(uk - I * 1E) = (A: - A)(Ak, - A)

*

(A: - A).

For it follows from the formulas (A.6.10.1) that U k ( b i ) = A!bi

+

bi+,+ . *

*

+ apb,

and the result therefore follows from (A.6.10).

(1 < i < n )

7 MINORS OF A DETERMINANT

377

(A.6.12) The endomorphism u is a nilpotent element of the ring End(E) and on1.y if all its eigenvalues are zero.

If u is nilpotent, it follows from (A.6.11) that all the eigenvalues of u are zero. Conversely, if all the ,Ii are zero, the formula (A.6.10.1) shows, by induction on k , that uk(E) c Kbk+ + K b , + , + * * + Kb, if k < n, and finally that u"(E) = {0}, that is to say, u" = 0.

-

7. M I N O R S OF A D E T E R M I N A N T

(A.7.1) Let E be a vector space of dimension n over K, and let ( b i ) l d i 6 n be a basis of E. For each subset I of the index set A = { 1, 2, . . . , n } , let E(I) be the subspace of E generated by the 6 , with i E I . Then E is the direct sum of E(I) and E(A - I). If I = { i l , i , , . . . , ir}, where i , < iz < ... < i,, let j , be the bijection of K' onto E(I) such that j , ( e k )= b,, (1 < k < r ) , where (ek)l4 k G r is the canonical basis of K' (A.4.4). Also letp, be the linear mapping of E onto K' such that p,(bi,) = ek for 1 < k < r, and p,(bj) = 0 if j $ I. The kernel ofp, is therefore E(A - I), and the restriction ofp, to E(1) is a bijection of E(1) onto K'. (A.7.2) Let u be an endomorphism of E and let M ( u ) = ( a j i ) be its matrix with respect to the basis (bi) (A.6.8). If I , J are two subsets of the index set A, having the same number of elements r, consider the endomorphism uJ, = p u 0 j , of K'. Its matrix with respect to the canonical basis (e,) of K' consists of those a j i for which i E I and j~ J. The determinant of this matrix (that is to say, det(u J l ) ) is called the r x r minor of det(u), corresponding to the basis ( b J l i Q n of E and the subsets I, J of the index set A. 0

(A.7.3) An endomorphism u of E is of rank r i f and only i f all the s x s minors (where s > r ) in det(u) relatiue to (bi)are zero and at least one of the r x r minors is nonzero.

Let p be the rank of u. With the notation of (A.7.2), we have ujI(K') = pj(u(E(I))), hence (A.4.18)rank(uJ I ) = dim(uJ1(K')) < dim(u(E(1))) < dim u(E) = rank(u) = p . If r > p , we therefore have det(u,,) = 0, by (A.4.19) and (A.6.7). On the other hand, there exists a subset I, of A, containing p elements, such that E(1,) is supplementary to ker(u), and a subset J, of A containing p elements, such that E(A - J,) is sumlementarv to u(E) (A.4.5).

378

APPENDIX: ELEMENTS OF LINEAR ALGEBRA

It follows that u, restricted to E(To), is a bijection of E(Io) onto u(E) (A.4.19), and that p J orestricted to u(E) is a bijection of u(E) onto KP (A.3.5). Hence u~~~~ is bijective and therefore det(uJo,,) # 0 (A.6.7). The proposition follows immediately from these remarks. (A.7.4) With the preceding notations let us now take 1 = J = { 1, 2, . . . , m}, hence A - I = A - J = {m + 1, . . . , n } , and let us suppose in addition that uA-,,I = 0, in other words the matrix M(u) has the form

where X = M(u,,) is an m x m matrix, Y = M(u,,A - ,) an m x (n - m) matrix and Z = M(u,-,, A-I) an (n - m) x (n - m ) matrix (0 standing for the zero ( n - m) x m matrix). Then we have det(M(u)) = det(X) det(Z).

(A.7.4.1)

For if u is the endomorphism of E(1) having X as matrix with respect to (bi)l i 6 m , we have, with the notations of (A.6.10), f(u(b,),* . . > u(bm), u(bm+l), . * . 4bn)) = f ( v ( b l ) , * * * v(b,n), u(b,n+1), . . . 4bn)). 7

9

But the mapping ( ~ 91 *

* * 9

xm)

-tf(xl, * . ., x m

9

u(bm+1),

*

., u(bn))

is an alternating m-linear form on (E(I))m, hence, by (A.6.5), f(v(bl),

*

. ., u ( b m ) ,

. . u(bn)) X ) f ( b i , . . . , b,, , u ( b m + 1 ) , . . ., u(bn))*

u(bm+l)?

= (det

'9

For each j 2 rn + 1, let us write u(bj) = CJ + cy, with c j E E(I), c; By definition of an alternating multilinear form, we have

E

E(A-I).

. ., b m 4 b m + I), . . ., u(bn)) =f(b1, . . ., b m c;+1, . c:)* Let )Y then be the endomorphism of E(A - 1) having 2 as matrix with respect f(b1, *

9

* *

9

9

to ( b j ) m + l Q j Qby n ;definition w(bj) = cj" f o r j e A - I. The mapping ( x m + 1,

* *

9

xn) +f(b1,

*

. ., brn

is an alternating ( n - m)-linear form on (E(A f(b,,

. . . ,b,,

9

Xm + 1,

.. ., xn)

- I))"-",

~ ( b , + ~.). ., , ~ ( b , ) = ) (det Z)f(bl,

hence we get similarly

. . . , b,)

= det 2

which proves (A.7.4.1). By induction on r, we conclude that for any " triangular

7 MINORS OF A DETERMINANT

matrix of matrices ”

where X i j is an m i x mj matrix, we have (A .7.4.2)

det 0 = (det XII)(det X 2 * )

(“ Computation of a determinant by blocks ”).

(det XrV)

379

REFERENCES

[ I ] Ahlfors, L., “Complex Analysis.” McGraw-Hill, New York, 1953. [2] Bachmann, H., “Transfinite Zahlen” (Ergebnisse der Math., Neue Folge, Heft I). Springer, Berlin, 1955. [3] Bourbaki, N., “ Elements de Mathkmatique,” Livre I, “Theorie des Ensembles ” (Actual. Scient. Ind., Chaps. I, 11, No. 1212; Chap. 111, No. 1243). Hermann, Paris, 1954-1956. [4] Bourbaki, N., “ Elements de Mathkmatique,” Livre 11, “Algebre,” Chap. 11 (Actual. Scient. Ind., Nos. 1032, 1236, 2nd ed.). Herrnann, Paris, 1955. [5] Bourbaki, N., “ Elkments de Mathkmatique,” Livre 111, “Topologie gknkrale” (Actual. Scient. Ind., Chaps. I, 11, Nos. 858, 1142, 4th ed.; Chap. IX, No. 1045, 2nd ed.; Chap. X, No. 1084, 2nd ed.). Hermann, Paris, 1949-1958. 161 Bourbaki, N . , “Elements de Mathkmatique,” Livre V, “ Espaces vectoriels topologiques” (Actual. Scient. Ind., Chap. 1, 11, No. 1189, 2nd ed.; Chaps. 111-V, No. 1229). Hermann, Paris, 1953-1955. [7] Cartan, H., “Seminaire de I’Ecole Normale Superieure, 1951-1952: Fonctions analytiques et faisceaux analytiques.” [8] Cartan, H., “Theorie elementake des fonctions analytiques.” Hermann, Paris, 1961. [9] Coddington, E., and Levinson, N., “Theory of Ordinary Differential Equations.” McGraw-Hill, New York, 1955. [lo] Courant, R., and Hilbert, D., “Methoden der mathematischen Physik,” Vol. I, 2nd ed. Springer, Berlin, 1931. [ I I] Halmos, P., “Finite Dimensional Vector Spaces,” 2nd ed. Van Nostrand, Princeton, New Jersey, 1958. [I21 Ince, E., “Ordinary Differential Equations.” Dover Publications, New York, 1949. [I31 Jacobson, N., “Lectures in Abstract Algebra,” Vol. 11, “Linear Algebra.” Van Nostrand, Princeton, New Jersey, 1953. [I41 Kamke, E., “Differentialgleichungen reeller Funktionen.” Akad. Verlag, Leipzig, 1930. [I51 Kelley, J., “General Topology.” Van Nostrand, Princeton, New Jersey, 1955. [I61 Landau, E., “Foundations of Analysis.” Chelsea, New York, 1951. [ 171 Springer, G . , “Introduction to Riemann Surfaces.” Addison-Wesley, Reading, Massachusetts, 1957. [I81 Weil, A,, “Introduction a I’ktude des varietes kahleriennes” (Actual. Scient. Ind., No. 1267). Hermann, Paris, 1958. [I91 Weyl, H., “Die Idee der Riemannschen Flache,” 3rd ed. Teubner, Stuttgart, 1955. 380

INDEX

In the following index the first reference number refers to the number of the chapter in which the subject may be found and the second to the section within the chapter. A

Abel’s lemma: 9. I Abel’s theorem: 9.3, prob. 1 Absolute value of a real number: 2.2 Absolute value of a complex number: 4.4 Absolutely convergent series: 5 . 3 Absolutely summable family, absolutely sumniable subset: 5.3 Adjoint of an operator: 11.5 Algebraic multiplicity of an eigenvalue: I I .4 Amplitude of a complex number: 9.5, prob. 8 Analytic mapping: 9.3 Approximate solution of a differential equation: 10.5 Ascoli’s theorem: 7.5 At most denumerable set, at most denumerable family: I .9 Axiom of Archimedes: 2.1 Axiom of choice: 1.4 Axiom of nested intervals: 2.1

B Banach space: 5.1 Basis for the open sets of a metric space: 3.9 Belonging to a set: 1 . 1 Bergman’s kernel: 9.13, prob. Bessel’s inequality: 6.5 Bicontinuous mapping: 3.12

Bijective mapping, bijection: 1.6 Bloch’s constant: 10.3, prob. 5 Bolzano’s theorem: 3.19 Borel’s theorem: 8.14, prob. 4 Borel-Lebesgue axiom: 3.16 Borel-Lebesgue theorem : 3. I7 Boundary conditions for a differential equation: 11.7 Bounded from above, from below (subset of R): 2.3 Bounded subset of R : 2.3 Bounded real function: 2.3 Bounded set in a metric space: 3.4 Broken line: 5.1, prob. 4 Brouwer’s theorem for the plane: 10.2, prob. 3

C Canonical decomposition of a vector relatively to a hermitian compact operator: 11.5 Cantor’s triadic set: 4.2, prob. 2 &-Capacityof a set: 3.16, prob. 4 Cartesian product of sets: 1.3 Cauchy’s conditions for analytic functions: 9.10 Cauchy criterion for sequences: 3.14 Cauchy criterion for series: 5.2 Cauchy’s existence theorem for differential equations: 10.4 MI

382

INDEX

Cauchy’s formula : 9.9 Cross section of a set: 1.3 Cauchy’s inequalities: 9.9 Cut of the plane: 9.Ap.3 Cauchy-Schwarz inequality: 6.2 Cauchy sequence: 3.14 D Cauchy’s theorem on analytic functions: 9.6 Center of a ball: 3.4 Decreasing function: 4.2 Center of a polydisk: 9.1 Degenerate hermitian form: 6.1 Change of variables in an integral: 8.7 Dense set in a space, dense set with respect Circuit: 9.6 t o another set: 3.9 Closed ball: 3.4 Denumerable set, denumerable family: 1.9 Closed interval: 2.1 Derivative of a mapping at a point: 8.1 Closed polydisk: 9.1 Derivative in an open set: 8.1 Closed set: 3.8 Derivative of a function of one variable: 8.4 Closure of a set: 3.8 Derivative with respect to a subset of R:8.4 Cluster point of a set: 3.8 Derivative on the left, on the right: 8.4 Cluster value of a sequence: 3.13 Derivative (second, pth): 8.12 Codimension of a linear variety: 5.1, Derivative (pth) with respect to an interval: prob. 5 8.12 Coefficient (nth) with respect to an ortho- Diagonal : 1.4 normal system: 6.5 Diagonal process : 9.13 Commutatively convergent series: 5.3, Diameter of a set: 3.4 Difference of two sets: 1.2 prob. 4 Differentiable mapping at a point, in a set: Compact operator: 11.2 8.1 Compact set: 3.17 Differentiable with respect to the first, Compact space: 3.16 second, . . . , variable: 8.9 Complement of a set: 1.2 Differentiable (twice, p times): 8.12 Complete space: 3.14 Differential equation: 10.4 Complex number: 4.4 Dimension of a linear variety: 5.1, prob. 5 Complex vector space: 5.1 Dini’s theorem: 7.2 Composed mapping: 1.7 Direct image: 1.5 Condensation point: 3.9, prob. 4 Conformal mapping theorem: 10.3, prob. 4 Dirichlet’s function: 3.1 1 Disk: 4.4 Conjugate of a complex number: 4.4 Connected component of a set, of a point Discrete metric space: 3.2 and 3.12 Distance of two points: 3.1 in a space: 3.19 Distance of two sets: 3.4 Connected set, connected space: 3.19 Constant mapping: 1.4 Contained in a set, containing a set: 1.1 E Continuity of the roots as function of parameters: 9.17 Eigenfunction of a kernel function: 11.6 Continuous, continuous at a point: 3.11 Eigenspace corresponding to an eigenvalue: Continuously differentiable mapping: 8.9 11.1 Convergence radius of a power series: 9.1, Eigenvalue of an operator: 1 1.1 prob. 1 Eigenvalue of a Sturm-Liouville problem: Convergent sequence: 3.13 11.7 Convergent series: 5.2 Eigenvector of an operator: 1 I . 1 Convex set, convex function: 8.5, prob. 8 Eilenberg’s criterion: 9.Ap.3 Coordinate (nth) with respect to an ortho- Element: 1.1 normal system: 6.5 Elementary solution for a Sturm-Liouville problem: 11.7 Covering of a set: 1.8

INDEX

Empty set: 1.1 Endless road: 9.12, prob. 3 Entire function: 9.3 &-Entropyof a set: 3.16, prob. 4 Equation of a hyperplane: 5.8 Equicontinuous at a point, equicontinuous : 7.5 Equipotent sets: 1.9 Equivalence class, equivalence relation: 1.8 Equivalent norms: 5.6 Equivalent roads: 9.6 Essential mapping: 9.Ap.2 Essential singular point, essential singularity: 9.15 Euclidean distance: 3.2 Everywhere dense set: 3.9 Exponential function: 4.3 and 9.5 Extended real line: 3.3 Extension of a mapping: 1.4 Exterior point of a set, exterior of a set: 3.7 Extremity of an interval: 2.1 Extremity of a path: 9.6 F

Family of elements: 1.8 Finer distance, finer topology: 3.12 Finite number: 3.3 Fixed point theorem: 10.1 Fourier coefficient (nth): 6.5 Fredholm equation, Fredholm alternative: 11.6 Frobenius’s theorem: 10.9 Frobenius-Perron’s theorem : 11.l, prob. 6 Frontier point of a set, frontier of a set: 3.8 Full sequence of positive eigenvalues: 11.5, prob. 8 Function: 1.4 Function of bounded variation: 7.6, prob. 3 Function of positive type: 6.3, prob. 4 Functional graph, functional relation: 1.4 Functions coinciding in a subset: 1.4 Fundamental system of neighborhoods: 3.6 Fundamental theorem of algebra: 9.1 1

G Geometric multiplicity of a n eigenvalue: 11.4 Goursat’s theorem: 9.10, prob. 1

383

Gram determinant: 6.6, prob. 3 Graph of a relation: 1.3 Graph of a mapping: 1.4 Greatest lower bound: 2.3 Green function of a Sturm-Liouville problem: 11.7 Gronwall’s lemma: 10.5

H Haar orthonormal system: 8.7, prob. 7 Hadamard’s three circles theorem: 9.5, prob. 10 Hadamard’s gap theorem: 9.15, prob. 7 Hausdorff distance of two sets: 3.16, prob. 3 Hermitian form: 6.1 Hermitian kernel: 1I .6 Hermitian norm: 9.5, prob. 7 Hermitian operator: 11.5 Hilbert basis: 6.5 Hilbert space: 6.2 Hilbert sum of Hilbert spaces: 6.4 Homeomorphic metric spaces, homeomorphism: 3.12 Homogeneous linear differential equation: 10.8 Homogeneous hyperplane: 5.8, prob. 3 Homotopic paths, homotopic loops, homotopy of a path into a path: 9.6 and 10.2, prob. 6 Hyperplane: 5.8 and 5.8, prob. 3 Hyperplane of support: 5.8, prob. 3 I

Identity mapping: 1.4 Image of a set by a mapping: 1.5 Imaginary part of a complex number: 4.4 Implicit function theorem: 10.2 Improperly integrable function along an endless road, improper integral: 9.12, prob. 3 Increasing function: 4.2 Increasing on the right: 8.5, prob. 1 Indefinitely differentiable mapping : 8.12 Index of a point with respect to a circuit, of a circuit with respect to a point: 9.8 Index of a point with respect to a loop: 9.Ap.l Induced distance: 3.10 Inessential mapping: 9.Ap.2

384

INDEX

Infimum of a set, of a function: 2.3 Infinite product of metric spaces: 3.20, prob. 7 Injection, injective mapping: 1.6 Integer (positive or negative): 2.2 Integral: 8.7 Integral along a road: 9.6 Integration by parts: 8.7 Interior point of a set, interior of a set: 3.7 Intersection of two sets: 1.2 Intersection of a family of sets: 1.8 Inverse image: 1.5 Inverse mapping : 1.6 Isolated point of a set: 3.10 Isolated singular point: 9.15 Isometric spaces, isometry: 3.3 Isomorphism of prehilbert spaces: 6.2 Isotropic vector: 6.1 1

Jacobian matrix, jacobian: 8.10 Janiszewski’s theorem: 9.Ap.3 Jordan curve theorem: 9.Ap.4 Juxtaposition of two paths: 9.6 K

Kernel function: 11.6 L Lagrange’s inversion formula: 10.2, prob. 10 Laurent series: 9.14 Least upper bound: 2.3 Lebesgue function (nth): 11.6, prob. 2 Lebesgue’s property: 3.16 Legendre polynomials: 6.6 and 8.14, prob. 1 Leibniz’s formula: 8.13 Leibniz’s rule: 8.11 Length of an interval: 2.2 Limitof a function, limit of a sequence: 3.13 Limit on the left, limit on the right: 7.6 Linear differential equation: 10.6 Linear differential equation of order n : 10.6 Linear differential operator: 8.13 Linear form: 5.8 Linear variety: 5.1, prob. 5

Linkedbyabroken line(points): 5.1, prob. 4 Liouville’s theorem: 9.1 1 Lipschitzian function: 7.5, prob. 12, and 10.5 Locally closed set: 3.10, prob. 3 Locally compact space: 3.18 Locally connected space: 3.19 Locally lipschitzian function: 10.4 Logarithm: 4.3 and 9.5, prob. 8 Loop: 9.6 and 10.2, prob. 6 Loop homotopy: 9.6 and 10.2, prob. 6 M

Majorant: 2.3 Majorized set, majorized function: 2.3 Mapping: 1.4 Maximal solution of a differential equation: 10.7, prob. 4 Maximinimal principle: 11.5, prob. 8, and 11.7, prob. 2 Mean value theorem: 8.5 Mercer’s theorem : 1 1.6 Meromorphic function: 9.17 Method of the gliding hump: I I .5, prob. 4, and 11.6, prob. 2 Metric space: 3.1 Minimal solution of a differential equation: 10.7, prob. 4 Minorant: 2.3 Minorized set, minorized function: 2.3 Minkowski’s inequality: 6.2 Monotone function: 4.2 Morera’s theorem: 9.10, prob. 2 N

Natural boundary: 9.15, prob. 7 Natural injection: 1.6 Natural mapping of X into X/R: 1.8 Natural ordering: 2.2 Negative number: 2.2 Negative real half-line: 9.5, prob. 8 Neighborhood: 3.6 Newton’s approximation method: 10.2, prob. 5 Nondegenerate hermitian operator: 11.5 Norm: 5.1 Normally convergent series, normally summable family: 7.1 Normed space: 5.1

INDEX

0 One-to-one mapping: 1.6 Onto mapping: 1.6 Open ball: 3.4 Open covering: 3.16 Open interval: 2.1 Open neighborhood: 3.6 Open polydisk: 9.1 Open set: 3.5 Operator: 11.1 Opposite path: 9.6 Order of an analytic function at a point: 9.15 Order of a linear differential operator: 8.13 Ordered pair: 1.3 Origin of an interval: 2.1 Origin of a path: 9.6 Orthogonal projection: 6.3 Orthogonal supplement: 6.3 Orthogonal system: 6.5 Orthogonal to a set (vector): 6.1 Orthogonal vectors: 6.1 Orthonormal system: 6.5 Orthonormalization: 6.6 Oscillation of a function: 3.14

385

Precompact set: 3.17 Precompact space: 3.16 Prehilbert space: 6.2 Primary factor: 9.12, prob. 1 Primitive: 8.7 Principle of analytic continuation: 9.4 Principle of extension of identities: 3.15 Principle of extension of inequalities: 3.1 5 Principle of isolated zeros: 9.1 Principle of maximum: 9.5 Product of a family of sets: 1.8 Product of metric spaces: 3.20 Product of norrned spaces: 5.4 Projection (first, second, ith): 1.3 Projections in a direct sum: 5.4 Purely imaginary number: 4.4 Pythagoras’s theorem: 6.2

Q Quasi-derivative, quasi-differentiable function: 8.4, prob. 4 Quasi-hermitian operator: 11.5, prob. 18 Quotient set: 1.8 R

P p-adic distance: 3.2 Parallel hyperplane: 5.8, prob. 3 Parseval’s identities: 6.5 Partial derivative: 8.9 Partial mapping: 1.5 Partial sum (nth) of a series: 5.2 Partition of a set: 1.8 Path: 9.6 and 10.2, prob. 6 Path reduced to a point: 9.6 Peano curve: 4.2, prob. 5, and 9.12, prob. 5 Peano’s existence theorem: 10.5, prob. 4 PhragmCn-Lindelof’s principle: 9.5, prob. 16 Picard’s theorem: 10.3, prob. 8 Piecewise linear function: 8.7 Point: 3.4 Pole of an analytic function: 9.15 Positive definite hermitian form: 6.2 Positive hermitian form: 6.2 Positive hermitian operator: 11.5 Positive number: 2.2 Power series: 9.1

Radii of a polydisk: 9.1 Radius of a ball: 3.4 Rational number: 2.2 Rank theorem: 10.3 Real line: 3.2 Real number: 2.1 Real part of a complex number: 4.4 Real vector space: 5.1 Reflexivity of a relation: 1.8 Regular frontier point for an analytic function: 9.15, prob. 7 Regular value for an operator: 11.1 Regularization: 8.12, prob. 2 Regulated function: 7.6 Relative maximum: 3.9, prob. 6 Relatively compact set: 3.17 Remainder (nth) of a series: 5.2 Reproducing kernel: 6.3, prob. 4 Residue : 9.15 Resolvent of a linear differential equation: 10.8 Restriction of a mapping: 1.4 Riemann sums: 8.7, prob. 1

386

INDEX

Riesz (F.)’s theorem: 5.9 Road: 9.6 Rolle’s theorem: 8.2, prob. 4 RouchB’s theorem: 9.17

S Scalar : 9.1 Scalar product: 6.2 Schoenflies’s theorem: 9.Ap., prob. 9 Schottky’s theorem: 10.3, prob. 6 Schwarz’s lemma: 9.5, prob. 6 Second mean value theorem: 8.7, prob. 2 Segment: 5.1, prob. 4, and 8.5 Self-adjoint operator: 1 1.5 Semi-open interval : 2.1 Separable metric space: 3.10 Separating points (set of functions): 7.3 Separating two points (subset of the plane): 9.Ap.3 Sequence: 1.8 Series: 5.2 Set: 1.1 Set of mappings : 1.4 Set of uniqueness for analytic functions: 9.4 Simple arc, simple closed curve, simple loop, simple path: 9.Ap.4 Simply connected domain: 9.7 and 10.2, prob. 6 Simply convergent sequence, simply convergent series: 7.1 Simpson’s formula: 8.14, prob. 10 Singular frontier point for an analytic function: 9.15, prob. 7 Singular part of an analytic function at a point: 9.15 Singular values of a compact operator: 11.5, prob. 15 Solution of a differential equation: 10.4 and 11.7 Spectral value, spectrum of an operator: 11.1 Sphere: 3.4 Square root of a positive hermitian compact operator: 11.5, prob. 12 Star-shaped domain: 9.7 Step function: 7.6 Stone-Weierstrass theorem: 7.3 Strict relative maximum: 3.9, prob. 6 Strictly convex function: 8.5, prob. 8

Strictly decreasing, strictly increasing, strictly monotone: 4.2 Strictly negative, strictly positive number: 2.2 Sturm-Liouville problem: 11.7 Subfamily: 1.8 Subsequence: 3.1 3 Subset: 1.4 Subspace: 3.10 Subspace of a normed space: 5.4 Substitution of power series in power series: 9.2 Sum of a family of sets: 1.8 Sum of a series: 5.2 Sum of an absolutely summable family: 5.3 Supremum of a set, of a function: 2.3 Surjection, surjective mapping: 1.6 Symmetric bilinear form: 6.1 Symmetry of a relation: I .8 System of scalar linear differential equations: 10.6

T Tangent mappings a t a point: 8.1 Tauber’s theorem: 9.3, prob. 2 Taylor’s formula: 8.14 Term (nth) of a series: 5.2 Theorem of residues: 9.16 Tietze-Urysohn extension theorem : 4.5 Titchmarsh’s theorem: 11.6, prob. 1 1 Topological direct sum, topological direct summand, topological supplement: 5.4 Topological notion: 3.12 Topologically equivalent distances: 3.12 Topology: 3.12 Total derivative: 8.1 Total subset: 5.4 Totally disconnected set: 3.19 Transcendental entire function: 9.15,prob. 3 Transitivity of a relation: 1.8 Transported distance: 3.3 Triangle inequality: 3.1 and 5.1 Trigonometric polynomials : 7.4 Trigonometric system: 6.5

U Ultrametric inequality: 3.8, prob. 4 Underlying real vector space: 5.1

INDEX

Uniformly continuous function: 3.1 1 Uniformly convergent sequence, uniformly convergent series: 7.1 Uniformly equicontinuous set: 7.5, prob. 5 Uniformly equivalent distances: 3.14 Union of two sets: 1.2 Union of a family of sets: 1.8 Unit circle: 9.5 Unit circle taken n times: 9.8

Vector space: 5.1 Volterra kernel : 11.6, prob. 8 W

Weierstrass’s approximation theorem: 7.4 Weierstrass’s decomposition: 10.2, prob. 8 Weierstrass’s preparation theorem: 9.17, prob. 4 Weierstrass’s theorem on essential singularities: 9.15, prob. 2

V

Value of a mapping: 1.4 Vector basis: 5.9, prob. 2

387

Z Zero of an analytic function: 9.15

Pure and Applied Mathematics A Series of Monographs and Textbooks Editors

Samuel Eilenberg and Hyman Baas Columbia University, New York

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