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DIFFERENTIAL EQUATIONS, DYNAMICAL SYSTEMS, AND AN INTRODUCTION TO CHAOS Morris W. Hirsch University of California, Berkeley Stephen Smale University of California, Berkeley Robert L. Devaney Boston University AMSTERDAM • BOSTON • HEIDELBERG • LONDON NEW YORK • OXFORD • PARIS • SAN DIEGO SAN FRANCISCO • SINGAPORE • SYDNEY • TOKYO Academic Press is an imprint of Elsevier
Academic Press is an imprint of Elsevier 225 Wyman Street, Waltham, MA 02451, USA The Boulevard, Langford Lane, Kidlington, Oxford, OX5 1GB, UK c 2013 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data Hirsch, Morris W., 1933- Differential equations, dynamical systems, and an introduction to chaos. — 3rd ed. / Morris W. Hirsch, Stephen Smale, Robert L. Devaney. p. cm. ISBN 978-0-12-382010-5 (hardback) 1. Differential equations. 2. Algebras, Linear. 3. Chaotic behavior in systems. I. Smale, Stephen, 1930– II. Devaney, Robert L., 1948– III. Title. QA372.H67 2013 515’.35–dc23 2012002951 British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. For information on all Academic Press publications visit our Website at www.elsevierdirect.com Printed in the United States 12 13 14 15 16 10 9 8 7 6 5 4 3 2 1
Preface to Third Edition The main new features in this edition consist of a number of additional explo- rations together with numerous proof simplifications and revisions. The new explorations include a sojourn into numerical methods that highlights how these methods sometimes fail, which in turn provides an early glimpse of chaotic behavior. Another new exploration involves the previously treated SIR model of infectious diseases, only now considered with zombies as the infected population. A third new exploration involves explaining the motion of a glider. This edition has benefited from numerous helpful comments from a variety of readers. Special thanks are due to Jamil Gomes de Abreu, Eric Adams, Adam Leighton, Tiennyu Ma, Lluis Fernand Mello, Bogdan Przeradzki, Charles Pugh, Hal Smith, and Richard Venti for their valuable insights and corrections. ix
Preface In the thirty years since the publication of the first edition of this book, much has changed in the field of mathematics known as dynamical systems. In the early 1970s, we had very little access to high-speed computers and computer graphics. The word chaos had never been used in a mathematical setting. Most of the interest in the theory of differential equations and dynamical systems was confined to a relatively small group of mathematicians. Things have changed dramatically in the ensuing three decades. Comput- ers are everywhere, and software packages that can be used to approximate solutions of differential equations and view the results graphically are widely available. As a consequence, the analysis of nonlinear systems of differential equations is much more accessible than it once was. The discovery of com- plicated dynamical systems, such as the horseshoe map, homoclinic tangles, the Lorenz system, and their mathematical analysis, convinced scientists that simple stable motions such as equilibria or periodic solutions were not always the most important behavior of solutions of differential equations. The beauty and relative accessibility of these chaotic phenomena motivated scientists and engineers in many disciplines to look more carefully at the important differen- tial equations in their own fields. In many cases, they found chaotic behavior in these systems as well. Now dynamical systems phenomena appear in virtually every area of sci- ence, from the oscillating Belousov–Zhabotinsky reaction in chemistry to the chaotic Chua circuit in electrical engineering, from complicated motions in celestial mechanics to the bifurcations arising in ecological systems. xi
xii Preface As a consequence, the audience for a text on differential equations and dynamical systems is considerably larger and more diverse than it was in the 1970s. We have accordingly made several major structural changes to this book, including: 1. The treatment of linear algebra has been scaled back. We have dispensed with the generalities involved with abstract vector spaces and normed lin- ear spaces. We no longer include a complete proof of the reduction of all n× n matrices to canonical form. Rather, we deal primarily with matrices no larger than 4× 4. 2. We have included a detailed discussion of the chaotic behavior in the Lorenz attractor, the Shil’nikov system, and the double-scroll attractor. 3. Many new applications are included; previous applications have been updated. 4. There are now several chapters dealing with discrete dynamical systems. 5. We deal primarily with systems that are C , thereby simplifying many of ∞ the hypotheses of theorems. This book consists of three main parts. The first deals with linear systems of differential equations together with some first-order nonlinear equations. The second is the main part of the text: here we concentrate on nonlinear systems, primarily two-dimensional, as well as applications of these systems in a wide variety of fields. Part three deals with higher dimensional systems. Here we emphasize the types of chaotic behavior that do not occur in planar systems, as well as the principal means of studying such behavior—the reduction to a discrete dynamical system. Writing a book for a diverse audience whose backgrounds vary greatly poses a significant challenge. We view this one as a text for a second course in differ- ential equations that is aimed not only at mathematicians, but also at scientists and engineers who are seeking to develop sufficient mathematical skills to analyze the types of differential equations that arise in their disciplines. Many who come to this book will have strong backgrounds in linear algebra and real analysis, but others will have less exposure to these fields. To make this text accessible to both groups, we begin with a fairly gentle introduction to low-dimensional systems of differential equations. Much of this will be a review for readers with a more thorough background in differential equations, so we intersperse some new topics throughout the early part of the book for those readers. For example, the first chapter deals with first-order equations. We begin it with a discussion of linear differential equations and the logistic popula- tion model, topics that should be familiar to anyone who has a rudimentary acquaintance with differential equations. Beyond this review, we discuss the logistic model with harvesting, both constant and periodic. This allows us to introduce bifurcations at an early stage as well as to describe Poincar´e maps
Preface xiii and periodic solutions. These are topics that are not usually found in elemen- tary differential equations courses, yet they are accessible to anyone with a background in multivariable calculus. Of course, readers with a limited back- ground may wish to skip these specialized topics at first and concentrate on the more elementary material. Chapters 2 through 6 deal with linear systems of differential equations. Again we begin slowly, with Chapters 2 and 3 dealing only with planar sys- tems of differential equations and two-dimensional linear algebra. Chapters 5 and 6 introduce higher dimensional linear systems; however, our emphasis remains on three- and four-dimensional systems rather than completely gen- eral n-dimensional systems, even though many of the techniques we describe extend easily to higher dimensions. The core of the book lies in the second part. Here, we turn our atten- tion to nonlinear systems. Unlike linear systems, nonlinear systems present some serious theoretical difficulties such as existence and uniqueness of solu- tions, dependence of solutions on initial conditions and parameters, and the like. Rather than plunge immediately into these difficult theoretical questions, which require a solid background in real analysis, we simply state the impor- tant results in Chapter 7 and present a collection of examples that illustrate what these theorems say (and do not say). Proofs of all of the results are included in the final chapter of the book. In the first few chapters in the nonlinear part of the book, we introduce important techniques such as linearization near equilibria, nullcline analysis, stability properties, limit sets, and bifurcation theory. In the latter half of this part, we apply these ideas to a variety of systems that arise in biology, electrical engineering, mechanics, and other fields. Many of the chapters conclude with a section called “Exploration.” These sections consist of a series of questions and numerical investigations dealing with a particular topic or application relevant to the preceding material. In each Exploration we give a brief introduction to the topic at hand and provide references for further reading about this subject. But, we leave it to the reader to tackle the behavior of the resulting system using the material presented ear- lier. We often provide a series of introductory problems as well as hints as to how to proceed, but in many cases, a full analysis of the system could become a major research project. You will not find “answers in the back of the book” for the questions; in many cases, nobody knows the complete answer. (Except, of course, you!) The final part of the book is devoted to the complicated nonlinear behav- ior of higher dimensional systems known as chaotic behavior. We introduce these ideas via the famous Lorenz system of differential equations. As is often the case in dimensions three and higher, we reduce the problem of com- prehending the complicated behavior of this differential equation to that of understanding the dynamics of a discrete dynamical system or iterated
xiv Preface function. So we then take a detour into the world of discrete systems, dis- cussing along the way how symbolic dynamics can be used to describe certain chaotic systems completely. We then return to nonlinear differential equations to apply these techniques to other chaotic systems, including those that arise when homoclinic orbits are present. We maintain a website at math.bu.edu/hsd devoted to issues regarding this text. Look here for errata, suggestions, and other topics of interest to teachers and students of differential equations. We welcome any contributions from readers at this site.
1 First-Order Equations The purpose of this chapter is to develop some elementary yet important examples of first-order differential equations. The examples here illustrate some of the basic ideas in the theory of ordinary differential equations in the simplest possible setting. We anticipate that the first few examples will be familiar to readers who have taken an introductory course in differential equations. Later examples, such as the logistic model with harvesting, are included to give the reader a taste of certain topics (e.g., bifurcations, periodic solutions, and Poincar´e maps) that we will return to often throughout this book. In later chapters, our treatment of these topics will be much more systematic. 1.1 The Simplest Example The differential equation familiar to all calculus students, = ax, dx dt is the simplest. It is also one of the most important. First, what does it mean? Here x = x(t) is an unknown real-valued function of a real variable t and (t) for the derivative). In addi- dx/dt is its derivative (we will also use x tion, a is a parameter; for each value of a we have a different differential or x Differential Equations, Dynamical Systems, and an Introduction to Chaos. DOI: 10.1016/B978-0-12-382010-5.00001-4 c 2013 Elsevier Inc. All rights reserved. 1
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