Accessible parts of the boundary for domains in metric measure spaces

Abstract

We prove in the setting of \(Q\)-Ahlfors regular PI-spaces the following result: if a domain has uniformly large boundary when measured with respect to the \(s\)-dimensional Hausdorff content, then its visible boundary has large \(t\)-dimensional Hausdorff content for every \(0<t<s\leq Q-1\). The visible boundary is the set of points that can be reached by a John curve from a fixed point \(z_{0}\in \Omega\). This generalizes recent results by Koskela-Nandi-Nicolau (from \(\mathbb R^2\)) and Azzam (\(\mathbb R^n\)). In particular, our approach shows that the phenomenon is independent of the linear structure of the space.