Dichotomies in Stability Theory

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629

w. A. Coppel

Dichotomies in Stability Theory

Springer-Verlag Berlin Heidelberg New York 1978

Author

W.A. Cappel Department of Mathematics Institute of Advanced Studies Australian National University Canberra, ACT, 2600/Australia

AMS Subject Classifications (1970): 34D05, 58F15 ISBN 3-540-08536-X Springer-Verlag Berlin Heidelberg New York ISBN 0-387-08536-X Springer-Verlag New York Heidelberg Berlin This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically those of translation, reprinting, re-use of illustrations, broadcasting, reproduction by photocopying machine or similar means, and storage in data banks. Under § 54 of the German Copyright Law where copies are made for other than private use, a fee is payable to the publisher, the amount of the fee to be determined by agreement with the publisher. © by Springer-Verlag Berlin Heidelberg 1978 Printed in Germany Printing and binding: Beltz Offsetdruck, Hemsbach/Bergstr. 2140/3140-543210

PREFACE

Several years ago I formed the view that dichotomies, rather than Lyapunov's characteristic exponents, are the key to questions of asymptotic behaviour for nonautonomous differential equations.

I still hold that view, in spite of the fact that

since then there have appeared many more papers and a book on characteristic exponents.

On the other hand, there has recently been an important new development

in the theory of dichotomies.

Thus it seemed to me an appropriate time to give an

accessible account of this attractive theory. The present lecture notes are the basis for a course given at the University of Florence in May, 1977.

I am grateful to Professor R. Conti for the invitation to

visit there and for providing the incentive to put my thoughts in order.

I am also

grateful to Mrs Helen Daish and Mrs Linda Southwell for cheerfully and carefully typing the manuscript.

CONTENTS

STABILITY

Lecture 4.

Lecture 8.

Appendix Notes

1

EXPONENTIAL AND ORDINARY DICHOTOMIES

10

DICHOTOMIES AND FUNCTIONAL ANALYSIS

20

ROUGHNESS

28

DICHOTOMIES AND REDUCIBILITY

38

CRITERIA FOR AN EXPONENTIAL DICHOTOMY

1+7

DICHOTOMIES AND LYAPUNOV FUNCTIONS

59

EQUATIONS ON

67

R AND ALMOST PERIODIC EQUATIONS

DICHOTOMIES AND THE HULL OF AN EQUATION

71+

THE METHOD OF PERRON

87

92 95

Subject Index

98

1.

STABILITY

A dichotomy, exponential or ordinary, is a type of conditional stability.

Let us

begin, then, by recalling some facts about unconditional stability. The classical definitions of stability and asymptotic stability, due to Lyapunov, are well suited for the study of autonomous differential equations.

For non-

autonomous equations, however, the concepts of uniform stability and uniform asymptotic stability are more appropriate. Let

x(t)

be a solution of the vector differential equation

(1)

f( t, x )

which is defined on the half­line

uniformly stable if for each any solution s

0

x(t)

0 S t
0

00

there is a corresponding

0

Ix Cs ) ­ x(t) I < E

Ix(t) ­

is said to