Cell-Like Equivalences and Boundaries of CAT(0) Groups

TL;DR

This paper investigates the uniqueness of boundaries of CAT(0) groups under cell-like equivalence, proposing a strategy to affirm this in general and demonstrating success in various specific cases.

Contribution

It introduces a new approach to assess boundary equivalence of CAT(0) groups and provides evidence supporting positive answers in multiple scenarios.

Findings

Strategy often succeeds in establishing cell-like equivalence

Boundaries of CAT(0) groups can be more similar than previously thought

Supports the conjecture that boundaries are uniquely determined up to cell-like equivalence

Abstract

In 2000, Croke and Kleiner showed that a CAT(0) group G can admit more than one boundary. This contrasted with the situation for word hyperbolic groups, where it was well-known that each such group admitted a unique boundary---in a very stong sense. Prior to Croke and Kleiner's discovery, it had been observed by Geoghegan and Bestvina that a weaker sort of uniquness does hold for boundaries of torsion free CAT(0) groups; in particular, any two such boundaries always have the same shape. Hence, the boundary really does carry significant information about the group itself. In an attempt to strengthen the correspondence between group and boundary, Bestvina asked whether boundaries of CAT(0) groups are unique up to cell-like equivalence. For the types of space that arise as boundaries of CAT(0) groups, this is a notion that is weaker than topological equivalence and stronger than shape equivalence. In this paper we explore the Bestvina Cell-like Equivalence Question. We describe a straightforward strategy with the potential for providing a fully general positive answer. We apply that strategy to a number of test cases and show that it succeeds---often in unexpectedly interesting ways.