Stationary measures and random walks on $\tilde{A}_2$-buildings

Abstract: Consider a non-elementary group action of a locally compact second countable group $G$ on a possibly non-discrete building $X$ of type $\tilde{A}_2$. We study $G$-equivariant measurable maps between a given $G$-boundary $(B, \nu_B)$ and the set of chambers of the spherical building at infinity. Using techniques from boundary theory, we prove that such a boundary map exists and is unique. As a consequence, we prove that if $\mu$ is an admissible probability measure on $G$, there is a unique $\mu$-stationary measure $\nu$ supported on the chambers at infinity of the spherical building at infinity. We use this result to study random walks induced by the $G$-action, and we prove that for any base-point $o \in X$, $(Z_n o )$ converges almost surely to a regular point of the boundary under a second moment assumption.