Harmonic measure and quantitative connectivity: geometric characterization of the $L^p$ solvability of the Dirichlet problem. Part II

TL;DR

This paper establishes a geometric characterization of the weak-$A_ Infinity$ condition for harmonic measure on domains with regular boundaries, linking it to connectivity and rectifiability, and thus advancing understanding of the Dirichlet problem.

Contribution

It proves that the weak-$A_ Infinity$ condition for harmonic measure is equivalent to boundary rectifiability and a connectivity condition, providing the first geometric characterization of this measure condition.

Findings

Weak-$A_ Infinity$ condition implies weak local John connectivity.

Boundary rectifiability combined with connectivity characterizes harmonic measure.

Results connect harmonic measure properties with geometric boundary conditions.

Abstract

Let $\Omega\subset\mathbb R^{n+1}$ be an open set with $n$-AD-regular boundary. In this paper we prove that if the harmonic measure for $\Omega$ satisfies the so-called weak-$A_\infty$ condition, then $\Omega$ satisfies a suitable connectivity condition, namely the weak local John condition. Together with other previous results by Hofmann and Martell, this implies that the weak-$A_\infty$ condition for harmonic measure holds if and only if $\partial\Omega$ is uniformly $n$-rectifiable and the weak local John condition is satisfied. This yields the first geometric characterization of the weak-$A_\infty$ condition for harmonic measure, which is important because of its connection with the Dirichlet problem for the Laplace equation.