Coarse Z-Boundaries for Groups

Abstract

We generalize Bestvina’s notion of a $\mathcal{Z}$-boundary for a group to that of a “coarse $\mathcal{Z}$-boundary”. We show that established theorems about $\mathcal{Z}$-boundaries carry over nicely to the more general theory, and that some wished-for properties of $\mathcal{Z}$-boundaries become theorems when applied to coarse $\mathcal{Z}$-boundaries. Most notably, the property of admitting a coarse $\mathcal{Z}$-boundary is a pure quasi-isometry invariant. In the process, we streamline both new and existing definitions by introducing the notion of a “model $\mathcal{Z}$-geometry”. In accordance with the existing theory, we also develop an equivariant version of the above—that of a “coarse $\mathit{E}\mathcal{Z}$-boundary”.