Finely Harmonic Functions

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289 Bent Fuglede Kebenhavns Universitet, K0benhavn/Danmark

Finely Harmonic Functions

Springer-Verlag Berlin· Heidelberg· NewYork 1972

AMS Subject Classifications (1970): 3102, 31B05, 31B15, 31B99, 31C05, 31D05

ISBN 3-540-06005-7 Springer-Verlag Berlin . Heidelberg . New York ISBN 0-387-06005-7 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 phorocopying 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 hy agreement with the publisher.

© by Springer-Verlag Berlin - Heidelberg 1972. Library of Congress Catalog Card Number 72-90194. Printed in Germany. Otlsetdruck: Julius Beltz, HemsbachlBergstr.

1

Introduction

14

Chapter I. Preliminaries 1. The cone of positive hyperharmonic functions

14

2. Semibounded potentials ..••. 3. Balayage of measures. Base, thinness, and fine topology . . . .

19

Chapter II. Capacity in axiomatic potential theory 4. 5. 6. 7.

The domination axiom . The set functions dm, and (:.co) The functionals R. Jk d-, and R. (x o) Balayage on intersections of finely closed sets

, 5

Chapter III. Finely harmonic and finely hyperharmonic functions 8. Definitions and examples 9. Fundamental properties 10. Finely superharmonic functions and fine potentials 11. Balayage and specific multiplication relative to a finely open set Chapter IV. Applications 12. Properties involving fine connectivity and balayage of measures . . . • . • . • 13. On the balayage of semibounded potentials 14. The fine Dirichlet problem . 15. An application to the study of the Choquet property References

25 32 32 40 48 61 67 67 76 100 118 146 146 162 170 178 186

Introduction

0.1.

Definitions.

a harmonic space

12

harmonic measure

permits us, in particular, to define the

V

relative to a finely 1) open set

V

and a point x

The theory of balayage of measures on

as follows

[V

e:c

that is, the swept-out of the Dirac measure

ex

on the com-

V .

plement of

It is therefore natural to introduce corresponding notions of "fine harmonicity" and "fine hyperharmonicity" for numerical functions

f

L1 ciL

defined in a finely open set

as follows

(Definitions 8.1 and 8.3 below):

f

A function

j

is finely lower semicontinuous and

the induced fine topology on finely open sets

V

f

[1

'> -

00

in

U ,

if

and if

has a basis consisting of

V

of fine closure

[t x) A function

LT

is called finely hyperharmonic in

[V

f dex

C [1 such that for every

is called finely harmonic in U

if

f

is

finite and finely continuous in [1, and if the finely open sets

....

V with V

C

U such that f

CV .

is Ex -lntegrable and

1) The qualification fine(1Y) refers to the fine topology on 12 , that is, the coarsest topology on 12 making all hyp