GTM216 matrices theory and applications (GTM 216 2002)

Matrices: Theory and Applications Denis Serre Springer Graduate Texts in Mathematics 216 Editorial Board S Axler F.W Gehring K.A Ribet This page intentionally left blank Denis Serre Matrices Theory and Applications Denis Serre Ecole Normale Supe´rieure de Lyon UMPA Lyon Cedex 07, F-69364 France Denis.SERRE@umpa.ens-lyon.fr Editorial Board: S Axler Mathematics Department San Francisco State University San Francisco, CA 94132 USA axler@sfsu.edu F.W Gehring Mathematics Department East Hall University of Michigan Ann Arbor, MI 48109 USA fgehring@math.lsa.umich.edu K.A Ribet Mathematics Department University of California, Berkeley Berkeley, CA 94720-3840 USA ribet@math.berkeley.edu Mathematics Subject Classification (2000): 15-01 Library of Congress Cataloging-in-Publication Data Serre, D (Denis) [Matrices English.] Matrices : theory and applications / Denis Serre p cm.—(Graduate texts in mathematics ; 216) Includes bibliographical references and index ISBN 0-387-95460-0 (alk paper) Matrices I Title II Series QA188 S4713 2002 512.9′434—dc21 2002022926 ISBN 0-387-95460-0 Printed on acid-free paper Translated from Les Matrices: The´orie et pratique, published by Dunod (Paris), 2001  2002 Springer-Verlag New York, Inc All rights reserved This work may not be translated or copied in whole or in part without the written permission of the publisher (Springer-Verlag New York, Inc., 175 Fifth Avenue, New York, NY 10010, USA), except for brief excerpts in connection with reviews or scholarly analysis Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights Printed in the United States of America SPIN 10869456 Typesetting: Pages created by the author in LaTeX2e www.springer-ny.com Springer-Verlag New York Berlin Heidelberg A member of BertelsmannSpringer Science+Business Media GmbH To Pascale and Joachim This page intentionally left blank Preface The study of matrices occupies a singular place within mathematics It is still an area of active research, and it is used by every mathematician and by many scientists working in various specialities Several examples illustrate its versatility: • Scientific computing libraries began growing around matrix calculus As a matter of fact, the discretization of partial differential operators is an endless source of linear finite-dimensional problems • At a discrete level, the maximum principle is related to nonnegative matrices • Control theory and stabilization of systems with finitely many degrees of freedom involve spectral analysis of matrices • The discrete Fourier transform, including the fast Fourier transform, makes use of Toeplitz matrices • Statistics is widely based on correlation matrices • The generalized inverse is involved in least-squares approximation • Symmetric matrices are inertia, deformation, or viscous tensors in continuum mechanics • Markov processes involve stochastic or bistochastic matrices • Graphs can be described in a useful way by square matrices viii Preface • Quantum chemistry is intimately related to matrix groups and their representations • The case of quantum mechanics is especially interesting: Observables are Hermitian operators, their eigenvalues are energy levels In the early years, quantum mechanics was called “mechanics of matrices,” and it has now given rise to the development of the theory of large random matrices See [23] for a thorough account of this fashionable topic This text was conceived during the years 1998–2001, on the occasion of ´ a course that I taught at the Ecole Normale Sup´erieure de Lyon As such, every result is accompanied by a detailed proof During this course I tried to investigate all the principal mathematical aspects of matrices: algebraic, geometric, and analytic In some sense, this is not a specialized book For instance, it is not as detailed as [19] concerning numerics, or as [35] on eigenvalue problems, or as [21] about Weyl-type inequalities But it covers, at a slightly higher than basic level, all these aspects, and is therefore well suited for a graduate program Students attracted by more advanced material will find one or two deeper results in each chapter but the first one, given with full proofs They will also find further information in about the half of the 170 exercises The solutions for exercises are available on the author’s site http://www.umpa.ens-lyon.fr/ ˜serre/exercises.pdf This book is organized into ten chapters The first three contain the basics of matrix theory and should be known by almost every graduate student in any mathematical field The other parts can be read more or less independently of each other However, exercises in a given chapter sometimes refer to the material introduced in another one This text was first published in French by Masson (Paris) in 2000, under the title Les Matrices: th´eorie et pratique I have taken the opportunity during the translation process to correct typos and errors, to index a list of symbols, to rewrite some unclear paragraphs, and to add a modest amount of material and exercises In particular, I added three sections, concerning alternate matrices, the singular value decomposition, and the Moore–Penrose generalized inverse Therefore, this edition differs from the French one by about 10 percent of the contents Acknowledgments Many thanks to the Ecole Normale Sup´erieure de Lyon and to my colleagues who have had to put up with my talking to them so often about matrices Special thanks to Sylvie Benzoni for her constant interest and useful comments Lyon, France December 2001 Denis Serre Contents Preface vii List of Symbols xiii Elementary Theory 1.1 Basics 1.2 Change of Basis 1.3 Exercises 1 13 Square Matrices 2.1 Determinants and Minors 2.2 Invertibility 2.3 Alternate Matrices and the Pfaffian 2.4 Eigenvalues and Eigenvectors 2.5 The Characteristic Polynomial 2.6 Diagonalization 2.7 Trigonalization 2.8 Irreducibility 2.9 Exercises 15 15 19 21 23 24 28 29 30 31 Matrices with Real or Complex Entries 3.1 Eigenvalues of Real- and Complex-Valued Matrices 3.2 Spectral Decomposition of Normal Matrices 3.3 Normal and Symmetric Real-Valued Matrices 40 43 45 47 188 10 Approximation of Eigenvalues 10.4.2 The Inverse Power Method Let us assume that M is invertible The standard power method, applied to M −1 , furnishes the eigenvalue of least modulus, whenever it is simple, or at least its modulus in the general case Since the inversion of a matrix is a costly operation, we involve ourselves with that idea only if M has already been inverted, for example if we had previously had to make an LU or a QR factorization That is typically the situation when one begins to implement the QR algorithm for M It might look strange to involve a method giving only one eigenvalue in the course of a method that is expected to compute the whole spectrum The inverse power method is thus subtle Here is the idea One begins by implementing the QR method, until one gets coarse approximations µ1 , , µn of the eigenvalues λ1 , , λn If one persists in the QR method, the proof of Theorem 10.2.1 shows that the error is at best of order σ k with σ = maxj |λj+1 /λj | When n is large, σ is in general close to and this convergence is rather slow Similarly, the method with Rayleigh translations, for which σ is replaced by σ(η) := maxj |(λj+1 − η)/(λj − η)|, is not satisfactory However, if one wishes to compute a single eigenvalue, say λp , with full accuracy, the power method, applied to M − µp In , produces an error on the order of θk , where θ := |λp − µp |/ minj=p |λj − µp | is a small number, since λp − µp is small In practice, the inverse power method is used mainly to compute an approximate eigenvector, associated to an eigenvalue for which one already has a good approximate value 10.5 Leverrier’s Method The method of Leverrier allows for the computation of the characteristic polynomial of a square matrix Though inserted in this Chapter, this method is not suitable for computing approximate values of the eigenvalues of a matrix First of all, it furnishes only the characteristic polynomial which, as mentioned at the opening if this chapter, is not a good technique for computing the eigenvalues Its interest is purely academic Observe, however, that it is of great generality, applying to matrices with entries in any field of characteristic 10.5.1 Description of the Method Let K be a field of characteristic and M ∈ Mn (K) be given Let us denote by λ1 , , λn the eigenvalues of M , counted with multiplicity Let us define the two following lists of n numbers: 10.5 Leverrier’s Method 189 Elementary symmetric polynomials σ1 := σ2 := λ1 + · · · + λn = Tr M, λj λk , j