# Comparing the Roller and B(X) boundaries of CAT(0) cube complexes

## TL;DR

This paper investigates conditions under which the subset B(X) of the Roller boundary of a CAT(0) cube complex X equals the entire boundary, with implications for geometric group actions and boundary properties.

## Contribution

It provides equivalent characterizations for when B(X) equals the entire Roller boundary and applies these to relate 2-dimensional complexes and Coxeter/Artin groups.

## Key findings

- B(X) equals the entire Roller boundary under specific conditions.
- 2-dimensional complexes are quasi-isometric to complexes with full Roller boundary.
- Certain Coxeter/Artin groups act on complexes with full Roller boundary.

## Abstract

The Roller boundary is a well-known compactification of a CAT(0) cube complex X. When X is locally finite, essential, irreducible, non-Euclidean and admits a cocompact action by a group G, Nevo-Sageev show that a subset, B(X), of the Roller boundary is the realization of the Poisson boundary and that the action of G on B(X) is minimal and strongly proximal. Additionally, these authors show B(X) satisfies many other desirable dynamical and topological properties. In this article we give several equivalent characterizations for when B(X) is equal to the entire Roller boundary. As an application we show, under mild hypotheses, that if X is also 2-dimensional then X is G-equivariantly quasi-isometric to a CAT(0) cube complex X' whose Roller boundary is equal to B(X'). Additionally, we use our characterization to show that the usual CAT(0) cube complex for which an infinite right-angled Coxeter/Artin group acts on geometrically has Roller boundary equal to B(X), as long as the corresponding group does not decompose as a direct product.