Potential Theory

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408 John Wermer

Potential Theo ry Second Edition

Springer-Verlag Berlin Heidelberg NewYork 1981

Author John Wermer Department of Mathematics Brown University Providence, RI 02912 U.S.A.

AMS Subject Classifications (1980); 31-02, 31BXX, 31B05. 31BlO, 31B15, 31B20 ISBN 3-540-10276-0 Springer-Verlag Berlin Heidelberg New York ISBN 0-387-10276-0 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 1981 Printed in Germany Printing and binding: Beltz Offsetdruck, Hemsbach/Bergstr. 2141/3140-543210

POTENTIAL THEORY John Wermer

CONTENTS

1. Introduction

VII

2. Electrostatics 3. Poisson's Equation

11

4. Fundamental Solutions

17

5. Capacity

26

6. Energy

34

7. Existence of the Equilibrium Potential

41

8. Maximum Principle for Potentials

50

9. Uniqueness of the Equilibrium Potential

56

10. The Cone Condition

60

11. Singularities of Bounded Harmonic Functions

66

12. Green's Functions

74

13. The Kelvin Transform

84

14. Perron's Method

91

15. Barriers

100

16. Kellogg's Theorem

108

17. The Riesz Decomposition Theorem

114

18. Applications of the Riesz Decomposition

129

19. Wiener's Criterion

138

Appendix

158

References

161

Bib liography

164

Index

166

1.

Introduction

Potential theory grew out of mathematical physics, in particular out of the theory of gravitation and the theory of electrostatics. Mathematical physicists such as Poisson and Green introduced some of the central ideas of the subject. A mathematician with a general knowledge of analysis may find it useful to begin his study of classical potential theory by looking at its physical origins.

Sections 2, 5 and 6 of these Notes give in part

heuristic arguments based on physical

These heuristic

arguments suggest mathematical theorems and provide the mathematician with the problem of finding the proper hypotheses and mathematical proofs. These Notes are based on a one-semester course given by the author at Brown University in 1971.

On the part of the reader, they assume a

knowledge of Real Function Theory to the extent of a first year graduate course.

In addition some elementary facts regarding harmonic functions

are assumed as known. the Appendix.

For convenience we have listed these facts in

Some notation is also explained there.

Essentially all the proofs we give in the Notes are for Euclidean 3-space

R3

and Newtonian potentials

Ix-YI In Section 4 we discuss the situation in needed to go from most results for

to n, R

(n > 3).

n f 3.

The modifications

n > 3, are merely technical, so we state

VIII

W