Vietoris-Rips complexes of affine Coxeter groups

Abstract

We prove that all affine Coxeter groups admit contractible Vietoris-Rips complexes. More precisely, for any affine Coxeter group $W_a$ there exists a finite generating set $A$ such that $\mathcal{VR}_t(W_a;A)$ is contractible for all $t\ge 1$. We prove this as a special case of a more general result about certain groups. Let $G$ be a group and $H$ a subgroup. A subgroup $F\le G$ is a supplement of $H$ in $G$ if $G=HF$. We prove that for $H\le G$ with $F$ a supplement of $H$ in $G$, for generating sets $A_H$ and $A_G$ of $H$ and $G$ satisfying certain conditions involving $F$, the Vietoris-Rips complexes $\mathcal{VR}_t(G;A_G)$ and $\mathcal{VR}_t(H;A_H)$ are homotopy equivalent for all $t\ge 1$. In particular this applies to affine Coxeter groups, since free abelian groups admit contractible Vietoris-Rips complexes. Beyond these general results, we also investigate affine Coxeter groups $W_a$ specifically using the standard generating set $S$, and prove contractibility of $\mathcal{VR}_t(W_a;S)$ for large enough $t$.