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IX - Minimal excessive measures and functions (Trans. AMS 258 (1980) 217–244)

Published online by Cambridge University Press:  18 March 2010

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Summary

ABSTRACT. Let H be a class of measures or functions. An element h of H is minimal if the relation h = h1 + h2, h1, h2H implies that h1, h2 are proportional to h. We give a limit procedure for computing minimal excessive measures for an arbitrary Markov semigroup Tt in a standard Borel space E. Analogous results for excessive functions are obtained assuming that an excessive measure γ on E exists such that Tf = 0 if f = 0 γ-a.e. In the Appendix, we prove that each excessive element can be decomposed into minimal elements and that such a decomposition is unique.

Introduction.

In 1941 R. S. Martin [13] published a paper where positive harmonic functions in a domain D of a Euclidean space were investigated. Let H stand for the class of all such functions subject to condition f(a) < ∞ where a is a fixed point of D. Martin has proved that:

  1. (a) each element of H can be decomposed in a unique way into minimal elements normalized by the condition f(a) = 1;

  2. (b) if the Green function of the Laplacian in D is known, then all minimal elements can be computed by a certain limit process.

J. L. Doob [2] has discovered that the Martin decomposition of harmonic functions is closely related to the behaviour of Brownian paths at the first exit time from D. G. A. Hunt [9] has shown that, using these relations, it is possible to get Martin's results by probabilistic considerations. Actually only discrete Markov chains were treated in [1] and [5], however, the methods are applicable to Brownian motion as well.

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Publisher: Cambridge University Press
Print publication year: 1982

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