Unavoidable sets and harmonic measures living on small sets

Abstract

Given a connected open set $U\ne \varnothing$ in $R^d$, $d\ge 2$, a relatively closed set $A$ in $U$ is called \emph{unavoidable in $U$}, if Brownian motion, starting in $x\in U\setminus A$ and killed when leaving $U$, hits $A$ almost surely or, equivalently, if the harmonic measure for $x$ with respect to $U\setminus A$ has mass $1$ on $A$. First a new criterion for unavoidable sets is proven which facilitates the construction of smaller and smaller unavoidable sets in $U$. Starting with an arbitrary champagne subdomain of $U$ (which is obtained omitting a locally finite union of pairwise disjoint closed balls $\overline{B}(z,r_z)$, $z\in Z$, satisfying $\sup_{z\in Z} r_z/\mbox{dist}(z,U^c)<1$), a combination of the criterion and the existence of small nonpolar compact sets of Cantor type yields a set $A$ on which harmonic measures for $U\setminus A$ are living and which has Hausdorff dimension $d-2$ and, if $d=2$, logarithmic Hausdorff dimension $1$. This can be done as well for Riesz potentials (isotropic $\alpha$-stable processes) on Euclidean space and for censored stable processes on $C^{1,1}$ open subsets. Finally, in the very general setting of a balayage space $(X,\mathcal{W})$ on which the function $1$ is harmonic (which covers not only large classes of second order partial differential equations, but also non-local situations as, for example, given by Riesz potentials, isotropic unimodal Lévy processes or censored stable processes) a construction of champagne subsets $X\setminus A$ of $X$ with small unavoidable sets $A$ is given which generalizes (and partially improves) recent constructions in the classical case.

27 pages, 2 figures

References in corpus (2)

• On Harnack inequality and Hölder regularity for isotropic unimodal Lévy processes
• Unavodiable collections of balls for isotropic Lévy processes

Cited by in corpus (6)

• Unavodiable collections of balls for isotropic Lévy processes
• Harnack inequalities for Hunt processes with Green function
• Equicontinuity of harmonic functions and compactness of potential kernels
• Liouville property, Wiener's test and unavoidable sets for Hunt processes
• Absolute continuity and singularity of probability measures induced by a purely discontinuous Girsanov transform of a stable process
• Nearly hyperharmonic functions are infima of excessive functions