Approximation Theory in the Central Limit Theorem

Mathematics and Its Applications (Soviet Series)

Managing Editor:

M. HAZEWINKEL Centre for Mathematics and Computer Science, Amsterdam, The Netherlands

Editorial Board:

A. A. KIRILLOV, MGU, Moscow, U.S.s.R. Yu. I. MANIN, Steklov Institute ofMathemalics, Moscow, U.S.s.R. N. N. MOISEEV, Computing Centre, Academy of Sciences, Moscow, U.s.s.R. S. P. NOVIKOV, Landau Institute of Theoretical Physics, Moscow, U.s.S.R. M. C. POLYVANOV, SteklovInstitute of Mathematics, Moscow, U.s.s.R. Yu. A. ROZANOV, Steklov Institute of Mathematics, Moscow, U.S.S.R.

Volume 32

Approximation Theory in the Central Limit Theorem Exact Results in Banach Spaces

by

v. Paulauskas and

A. Rackauskas Department of Mathematical Analysis, Vilnius University, Vilnius, U.s.s.R.

Kluwer Academic Publishers Dordrecht / Boston / London

Library of Congress Cataloging in Publication Data

Paulauskas, V. I. (Vigantas I~novich) Approximation theory in central limit theorems-­

exact results in Banach spaces.

(Mathematics and its applications. Soviet series) Translation of: Tochnost' a pproksirnat~ii v t'1entral' noi

peredel'not teoreme v banakhovykh prostranstvakh. Bibliography: p. Includes index. 1. Asymptotic distribution (Probability theory)

2. Banach spaces. 3. Central limit theorem. 4. Con­vergence. I. Rachkauskas, A. iD. (Alfredas IUrgevich) II. Title. III. Series: Mathematics and its applications (Kluwer Academic Publishers). Soviet series. QA273.6.P3813 1989 519.2 88-23101

ISBN-13: 978-94-011-7800-6 e-ISBN-13: 978-94-011-7798-6 DOl: 10.1007/978-94-011-7798-6

Published by Kluwer Academic Publishers, P.O. Box 17,3300 AA Dordrecht, The Netherlands.

Kluwer Academic Publishers incorporates the publishing programmes of D. Reidel, Martinus NiJboff, Dr W. Junk and MTP Press.

Sold and distributed in the U.S.A. and Canada by Kluwer Academic Publishers, 101 Philip Drive, Norwell, MA 02061, U.S.A.

In all other countries, sold and distributed by Kluwer Academic Publishers Group, P.O. Box 322, 3300 AH Dordrecht, The Netherlands.

Translated by B. Svecevicius and V. Paulauskas

Original publication: TOtlHOCTb annpOKCHMaU.HH B u.eHTpaJlbHOH npe.ll.eJlbHOH TeopeMe B 6aHaxoBblx rrpoCTpaHcTBax

published by Mokslas, Vilnius, © 1987

All Rights Reserved This English edition © 1989 Kluwer Academic Publishers Softcover reprint of the hardcover 1st edition 1989

No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical including photocopying, recording or by any information storage and retrieval system, without written permission from the copyright owner.

To our reviving Homeland LITHUANIA

Contents

Editor's Preface

Preface

Introduction

list of Notations

Chapter 1. General Questions of the Distribution Theory in Banach Spaces

1.1. Random Elements in Banach Spaces

1.2. Main Facts on Gaussian Measures in Banach Spaces

1.3. Weak Convergence of Probability Measures and Some Inequalities

Chapter 2. Some Questions of Non-linear Analysis

2.1. Differentiation in Normed Spaces

2.2. Smooth Approximation in a Banach Space

2.3. Supplements

Chapter 3. The Central limit Theorem in Banach Spaces

3.1. Operators of 'JYpe p

3.2. The Central Limit Theorem

3.3. Supplements

Chapter 4. Gaussian Measure of e-Strip of Some Sets

4.1. General Remarks

4.2. Distribution Density of the Norm of a Gaussian Element in ~

4.3. Distribution Density of the Norm of a Gaussian Element in Co

4.4. Supplements

ix

xi

xiii

xvii

5

7

11

11

22

41

44

44

52

59

62

62

63

67

7S

viii CONTENTS

Chapter S. Estimates of Rate of Convergence in the Central Umit Theorem 80

5.1. Estimates of the Rate of Convergence on Sets 80

5.2. Estimates of the Rate of Convergence in the Central Limit Theorem in Some Banach Spaces 11 0

5.3. Estimates of the Rate of Convergence in the Prokhorov Metric and the Bounded Lipschitz Metric 122

5.4. Supplements 135

Bibliographical Notes 139

References 144

Index 154

SERIES EDITOR'S PREFACE

~Et mai . ... , si j'avait su comment en revenir. je n'y serais point aIIe.'

Jules Verne

The series is divergent: therefore we may be able to do something with it.

O. Heaviside

One service mathematics has rendered the human race. It has put common sense back where it belongs, on the topmost shelf next to the dusty canister labelled 'discarded non-sense' .

Eric T. Bell

Mathematics is a tool for thought. A highly necessary tool in a world where both feedback and non­linearities abound. Similarly, all kinds of parts of mathematics serve as tools for other parts and for other sciences.

Applying a simple rewriting rule to the quote on the right above one finds such statements as: 'One service topology has rendered mathematical physics .. .'; 'One service logic has rendered com­puter science .. .'; 'One service category theory has rendered mathematics .. .'. All arguably true. And all statements obtainable this way form part of the raison d'etre of this series.

This series, Mathematics and Its Applications, started in 1977. Now that over one hundred volumes have appeared it seems opportune to reexamine its scope. At the time I wrote

"Growing specialization and diversification have brought a host of monographs and textbooks on increasingly specialized topics. However, the 'tree' of knowledge of mathematics and related fields does not grow only by putting forth new branches. It also happens, quite often in fact, that branches which were thought to be completely disparate are suddenly seen to be related. Further, the kind and level of sophistication of mathematics applied in various sciences has changed drastically in recent years: measure theory is used (non-trivially) in regional and theoretical economics; algebraic geometry interacts with physics; the Minkowsky lemma, coding theory and the structure of water meet one another in packing and covering theory; quantum fields, crystal defects and mathematical programming profit from homotopy theory; Lie algebras are relevant to filtering; and prediction and electrical engineering can use Stein spaces. And in addition to this there are such new emerging subdisciplines as 'experimental mathematics', 'CFD', 'completely integrable systems', 'chaos, synergetics and large-scale order', which are almost impossible to fit into the existing classification schemes. They draw upon widely different sections of mathematics."

By and large, all this still applies today. It is still true that at first sight mathematics seems rather fragmented and that to find, see, and exploit the deeper underlying interrelations more effort is needed and so are books that can help mathematicians and scientists do so. Accordingly MIA will continue to try to make such books available.

If anything, the description I gave in 1977 is now an understatement. To the examples of interaction areas one should add string theory where Riemann surfaces, algebraic geometry, modu­lar functions, knots, quantum field theory, Kac-Moody algebras, monstrous moonshine (and more) all come together_ And to the examples of things which can be usefully applied let me add the topic 'finite geometry'; a combination of words which sounds like it might not even exist, let alone be applicable. And yet it is being applied: to statistics via designs, to radar/sonar detection arrays (via finite projective planes), and to bus connections of VLSI chips (via difference sets). There seems to be no part of (so-called pure) mathematics that is not in immediate danger of being applied. And, accordingly, the applied mathematician needs to be aware of much more_ Besides analysis and numerics, the traditional workhorses, he may need all kinds of combinatorics, algebra, probability, and so on.

In addition, the applied scientist needs to cope increasingly with the nonlinear world and the

x SERIES EDITOR'S PREFACE

extra mathematical sophistication that this requires. For that is where the rewards are. Linear models are honest and a bit sad and depresaing: proportional efforts and results. It is in the non­linear world that infinitesimal inputs may result in macroscopic outputs (or vice versa). To appreci­ate what I am hinting at: if dcctronics were linear we would have no fun with transistors and com­puters; we would have no TV; in fact you would not be reading these lines.

1bere is also no safety in ignoring such outlandish things as nonstandard analysis, superspace and anticommuting integration, p-adic and ultrametric space. All thm: have applications in both dectrical engineering and physics. Once, complex numbers were equally outlandish, but they fre­quently proved the shortest path between 'rea!' results. Similarly, the first two topics named have alIcady provided a number of 'wormhole' paths. There is no telling where all this is leading -fortunatdy.

Thus the original scope of the series, which for various (sound) reasons now comprises five sub­series: white (Japan), ydiow (China), red (USSR), blue (Eastern Europe), and green (everything dse), stilI applies. It has been enlarged a bit to include books treating of the tools from one subdis­cipline which are used in others. Thus the series stilI aims at books dealing with:

- a central concept which plays an important role in several different mathematical and/or scientific specialization areas;

- new applications of the results and ideas from one area of scientific endeavour into another; - influences which the results, problems and concepts of one fidd of enquiIy have, and have had,

on the devdopment of another.

The laws of large numbers, or, better, central limit theorems for sums of identically distributed random variables, are basic to (both applications and foundations of) probability and statistics. Given the frequent and natural role of Hilbert and Banach spaces as the spaces of values for such random variables, the question of how wdl these sums are approximated by Gaussian distributions becomes of fundamental importance. The present volume presents an up-to-date unified treatment of this topic for the case of Banach spaces.

The complementary topic of rates of convergence to the Gaussian limit law and the topic of large deviations in a HilbeJ:t space is the subject of a forthcoming monograph by Prokhorov, Sazo­nov and Jurinsky (1990, translation in this series 1991). Together the two books should give the reader a very good idea of what is known at present concerning these matters.

The shortest path between two uuths in the

real domain palset through the complex cIomaiD.

J. Had.mard

La physique De nous doone pas seuJ_t !'occ:uion de raoudre des probIanes ••. eIIe IIOUa fait pRllClltir Ia soIutica.

H. PoiDcut

Bussum, June 1989

Never lend books, for no one ever returns

them; the only book. I have in my library arc books that other folk have lent me.

Anatole France

The fuDCtion of an expert is not to be more right than other peop1c, but to be wrong for more sophisticated rasoDS.

David Butler

Michid Hazewinkd

Preface

The intention of this book is to analyse the accuracy in approximating the distribution of sums of independent identically distributed random elements in a Banach space by Gaussian distributions. During the past two decades quite a number of results, resembling the final ones, have been obtained in this field. At the same time a close relation has been found between the estimates of the rate of convergence in the central limit theorem and some other branches of mathematics (in particular, non-linear analysis). We have tried to systematize the known results by presenting them in a unified form. An 'operator language' is used for this purpose.

Due to limited space, it was not possible to touch upon certain trends in the asymptotic analysis of distributions of random element sums such as the rate of convergence to stable laws, or asymptotic expansions in the central limit theorem, approaching infinitely divisible laws in Banach spaces. In the future, the above questions, no doubt, may comprise a separate book.

Even concerning rates of convergence to the Gaussian law, not all questions have been discussed to the same extent. It is the hope to the authors, however, that the present book and the forthcoming monograph of I.V. Prokhorov, V.V. Sazonov and v.v. Iurinskii on the estimates of convergence rate and large deviations in a Hilbert space will provide the reader with a present-day review of research in this field.

The book consists of five chapters. Each chapter is divided into sections. At the end of Chapters 2 - 5 'Supplements' are provided which present short formulations of the results not included in the main text, but closely related to it.

The sign 0 denotes the end of a proof. The following abbreviations are used in the text: a.s. = almost surely; i.i.d. = independent identically distributed; r.v. = random variable; CLT = the central limit theorem, B-r.e. = random element in the space B. In each chapter theorems, lemmas, propositions and formulas are marked by two numbers: (4.3), for instance, denotes formula number 3 in Section 4 of the given chapter. If a different chapter is referred to, three numbers are used, e.g. (5.4.3), the first number denoting the number of the chapter.

The authors' express their sincere thanks to their teachers, Prof. I. Kubilius and Prof. V. Statulevicius, for constant attention to the present work.

We also thank Dr. v.Yu. Bentkus who read all the proofs with the utmost care and gave a number of remarks that contributed much to the improvement of the book. Last, but not least, we express our gratitude to allow colleagues who helped us prepare the book for publication.

Introduction

The problem of estimating the rate of convergence in the central limit theorem in Banach spaces has arisen following Yu.V. Prokhorov's fundamental work [167], which has proved to be of great significance to many subjects concerning limit theorems in infinite-dimensional spaces. It should be noted that, at that time, investigations of estimates of convergence rates in the central limit theorem in finite-dimensional Euclidean spaces were but in their infancy. It is, therefore, not surprising that the early estimates in Hilbert space obtained by N.N. Vakhanya and N.P. Kandelaki [214], [97], by use of the finite-dimensional approximation method followed by a transition to the limit in dimension, had a logarithmic order of decrease with respect to the number of summands.

A considerable increase in interest in estimates of the convergence rate in general Banach spaces was particularly stimulated by the appearance of the works of Yu.v. Prokhorov and V.V. Sazonov [168], [187], which showed that the problem of approximation accuracy in statistical distributions may be formulated as a problem on the estimate of the convergence rate in the central limit theorem in some Banach or Hilbert space for summands of a special structure. In particular, for such summands in Hilbert space, a power estimate of the convergence rate was obtained by the use of a finite-dimensional approximation [187], [186]. A great impetus in this field was provided by the work by J. Kuelbs and T. Kurtz [103]. The method suggested by these authors is no longer associated with finite-dimensional approximation, and the estimate of the rate of convergence in the central limit theorem in Hilbert space under natural moment conditions has a power order.

During the past decade, considerable progress has been achieved in estimates of the convergence rate. Some trends have been identified and the results obtained for some problems are similar to the final ones. Methods of research based on functional analysis have been devised, and regularities have been revealed which relate only to infinite-dimensional spaces and are conditioned by their geometry.

At present, to our belief, the following three trends of research in asymptotic analysis of sums of independent random elements in Banach space may be distinguished.

1. Estimate of convergence rate on sets and construction of asymptotic expansions, in view of sufficient smoothness of a set or a functional under consideration.

2. Estimate of convergence rate on sets with a non-smooth boundary in general Banach spaces.

3. Estimate of convergence rate in different metrics.

The present book is intended mainly to address the second and third group of problems, as it is to these problems that the studies of Vilnius mathematicians have given the greatest attention.

The idea on which the method of compositions is based is simple. It can be explained without any complex notations. Let us suppose that we must estimate the difference of two measures F (distribution of a sum) and G (limit distribution) on a certain subset A of a Banach space B, i.e. we must estimate the quantity

I F(A) - G(A) I = I~ XA(X)(F - G)(dt)l,

where XA is the indicator function of the set A. The first problem is to substitute the

xiv INfRODUCTION

discontinuous function X4 by the sufficiently smooth function ga,. which coincides with X4 everywhere, with the exception of the set

(aA). = {x EB: inf IIx - y II < £} yEM

of the £-neighbourhood, of the boundary aA of the setA. Here £ > 0 is a certain parameter, defining the accuracy of approximation that is chosen in the last step of estimating the quantity I F (A) - G (A) I . At this substitution two terms are obtained: the integral of the smooth function

I~ KA,.(x)(F - G)(dr) I, which is usually estimated by expanding the function KA,. into a Taylor series, and the quantity G « aA ).) whose estimate is the second problem.

A part of this book is devoted to the study of the above problems, on whose solution depends the rate of convergence in the central limit theorem.

The book contains an introduction and five chapters followed by bibliographical remarks and references.

The first chapter is auxiliary: definitions, concepts and statements of the theory of the distributions in Banach spaces necessary for further presentation are collected here.

The problem of constructing smooth functions 'well approximating' the indicator function of the given set in a Banach space is considered in the second chapter.

In addition to standard concepts of differentiation in the sense of Frechet and Gateaux, differentiation with respect to an operator has been considered. Though this conception is formally equivalent to the conception of differentiation in directions from a certain subspace, 'operator language' has been chosen in the present work as it is found to be preferable to the 'language of embeddings'.

The central limit theorem in Banach spaces is considered in the third chapter. Ever since the term 'central limit theorem' appeared, this notion was understood to be the convergence of distributions of sums of independent identically distributed summands to the Gaussian law. At present, by this notion we mean convergence of the distributions of sums If;'!.1 Xiii, where {Xiii, i = 1, ... , k,,}, n ~ 1, is a sequence of series of independent random variables or elements which are uniformly asymptotically negJ.ig1ble. The studies under such a general design in separable Banach spaces are considered in a monograph by A Araujo and E. Gine [3], where the central limit theorem in Banach spaces has been discussed in detail. In the present work, by the central limit theorem we mean a simple case of convergence of distributions of sums of LLd random elements to the Gaussian law. This is done for two reasons: firstly, the presentation of the general scheme would take up twice as much space in the book; and, secondly, many facts connecting the geometric properties of Banach space with limit theorems are easily seen in this simple situation. The main results in Chapter 3, the central limit theorem in spaces C(S) and ro in particular, are obtained by the use of type-2 operators. In the fourth chapter, the Gaussian measure of an £-strip of sets in a Banach space is considered. It is well known that in finite-dimensionaI space the Gaussian measure of an £­strip of a convex set admits a uniform estimate of the order £. But even in an infmite­dimensional Hilbert space, however, the class of ba1ls has no such property. This chapter presents mainly results confirming the essential difference in the problem under consideration in finite-dimensional and infinite-dimensionaI spaces. In particular, constructed is an example of a Gaussian r .e. in the space ('0, the distribution of the norm of which has unbounded density.

INTRODUCTION xv

Chapter 5, having taken up almost half of the book, is the main one. The estimates of the rate of convergence in the central limit theorem in Banach spaces are presented in this chapter.

The main result in Section 5.1 shows that for the summands with third finite moment, the estimate of convergence rate of the order O(n -1/6) on the sets with non-smooth boundary takes place under two conditions: (a) the condition for the behaviour of Gaussian measure of an e:-strip of sets, and (b) the existence of smooth approximation of a set's indicator function of a set. It is shown by examples that both conditions are necessary and are independent of each other. Besides, in the general case, the order O(n -1/6) is not improvable. This makes an essential difference in comparison to the case of the finite-dimensional Banach space.

It is in this section that a special method is proposed for estimating the convergence rate on sets for random elements that are mixtures of Gaussian measures. The section ends with the estimate of the rate of convergence on balls in a Hilbert space. When summands have finite third moment, the estimate has the order O(n -1/2) and is proved by the method of characteristic functions which was substantially improved by F. Gotze. At present this method is successfully used to obtain the estimates of convergence rates on smooth sets.

In Section 5.2 the estimates in classical Banach spaces C(S) and Co are considered. Special attention is given to the space qo, 1], which is of importance to both the theory of random processes and the theory of Banach spaces. In the case of the limit Wiener process, in particular, the estimate of convergence rate on the balls with centre at zero is of the order O(n- 1/ 6Inn).

In Section 5.3 the convergence rate in the Prokhorov metric and bounded Lipschitz metric is considered. The estimates of the rate of convergence are reduced to the problems of constructing smooth functions, and to finding the best approximations of functions by smooth functions. Examples confirm that in the general case the order of decrease with respect to the number of summands in the estimates obtained is not improved.

List of Notations

AC,A,A A. B B· BLa(B)

A complement, a closure, and an interior of a setA. A set of points the distance from which toA is less than E.

A normed or separable Banach space. A topologically conjugate space to B. A set of bounded real functions on B, satisfying the Holder condition with the exponent CIt a Banach space with respect to the norm

II/lIBLa = :~~If(x)1 + :pPyI/(x) -/CY)I 'lIx -YII-<>·

BL(B) = BL1(B). q})(B) A Borel u-algebra of a space B. CO(U, F) A class of continuous functions from U in F. CO(B) = CO(B, R). Cg(B) A class of continuous bounded real functions on B.

1111100 = sup I I (x) I· xE"B

C'(U, F) A class of k times continuously Frechet differentiable functions from U in F (p. 12).

C~(U, F) (p. 18). C""(U, F) A class of infinitely Frechet differentiable functions from U in F. C"" = C""(R) = C""(R, R). C(S) A Banach space of real continuous functions on the metric compact S,

IIxllC(s) = SUPSES Ix(s) I· cov X Covariance of r.e. X (p. 3). Co = {x = (x;, i ~ 1): lim;-+ooxi = O} A Banach space with respect to the norm

IIx II = sUPi>l I Xi I . Cd = {x E Co : Xi ~ 0, i ~ 1}. Co A = {x E Co: lim;-+ooXiA;-l = O}(~ = (,\;) E Cd) A Banach space with respect to the norm

. IIxll = SUPi>llxi I~i-l. C (., ., .) A constant dependent on the parameters in the parentheses. JY(U, F) A class of k times Frechet differentiable functions from U to F (p. 12). necu, F) (p. 18). £2('1) The domain of normal attraction of the Gaussian r.e. '1 (p. 80). 8s The Dirac measure defined by the point s; the element of space C· (S). aA The boundary of the setA. ~(e, '1;.<Ji) = SUPA EA I P{Sn(e) EA} - P{'1EA} I· An(X, Y; s1) (p. 83). {E;, i ~ 1} Rademacher sequence (p. 9). F(A, A 1) A special class of functions (p. 22). F A Banach space. H A separable Hilbert space with the norm 1'1 and the scalar product L .). H(S, p, x) A metric entropy (p. 6). I B An identical mappin,; of space B. K..,n = K..,n(E) = (E- 1/a, ... , E-n a) (p. 25). Lo(F) = Lo(o,ff. P, F) A space ofallF-r.e.

xviii

~(F) = lp(o,g; P, F) = {X E L,,(F) : E IIXIIP < 00}(0 < p < 00). Lp(F) = {X E lp(F) : EX = O}(I5,p < 00).

UST OF NOTATIONS

lp(S, /.I) A Banach space of equivalence classes of real measurable functions on S such that

11/11 = U If(x)IP/.I(c:k)(P <00 (l5,p <00).

Ln (B, F) A Banach space of n linear bounded mappings from B in F. L(B, F) = LI(B, F). L(e) A distribution of r.e. e. Lipp(F) = {fEcO(F): II/lIp:= SUPXfY If(x) - 1(Y) I p-l(x,y) < oo}. Lipa(F) = LipP(F) Wherep(x,y) = IIx _ylla. Lipp(S) A Banach space with respect to norm 1I/IILip,(s) = 1111100 + II/lI p, if S is a metric

compact. lp = {x = (Xi, i ~ 1): IIx II = (E~l IXi IP)1/P < oo}(l5,p < 00). 100 = {x = (Xi, i ~ 1) : IIx II = SUPi>1 I Xi I < oo}. Ioo,~ = {X E 100 : IIx II~. := SUPi~1 T Xi I ~i-l < oo}(~ E cti)· lipp(S) = {f E Lipp(S) : limp(s,t)-O I I (s) - f(t) I p-I(S, t) = O} A Banach space with respect

to the norm Il/IILip,(s) = 11/1100 + II/lI p, if S is a metric compact. lipa[O, 1] = Lipp(S), where S = [0, 1], p(s, t) = Is - f I a.

p. 0 r I A measure induced from p. by mapping f /.I3(e, ,,) A pseudomoment (p. 80). PCB) A set of probability measures on B. :>r'J(H, B) A set of -y-radonifying operators from H to B (p. 7). :>r(p., /.I) Prokhorov metric (p. 7). :>r(e, ,,) = :>r(L (e), L(,,». Rtc k-dimensional Euclidean space. R+ = [0,00). PBL(P., /.I) Bounded Lipschitzian metric (p. 7). PBtie, "2 = ~L(L (P, L (,,». . Qu' , Qu, Qu SpecIal classes of functIOns (p. 22). U2(B, F) A set of operators of type 2 from B in F (p. 45). Sn(e) = n -1/2(el + ... + en), where e, 6, ... , en are independent identically distributed

random elements. T'1 Covariance operator of r.e. fl. Vr(a) = {x: IIx - a II < r}. Vr = Vr(O). W;(f) = {x: f(x) < f}. Wa(B) Class of balls ofthe space B with centre in a (p. 34). XA (-) Indicator of the set A. 11'11 B Norm of the space B.

The sign to denote 'equals by definition'.