Property Gamma and McDuff Factors from Shift-Similar and Houghton-like Groups

Abstract

We initiate the study of von Neumann algebras of shift-similar and Houghton-like groups, introduced by Mallery and Zaremsky as analogs of self-similar and Röver-Nekrashevych groups, respectively. Given a shift-similar group $G$ of permutations of $\mathbb{N}$, one can form the Houghton-like group $H_n(G)$, a ``mashup" of the classical Houghton group $H_n$ with $G$, which acts on $X_n$, the disjoint union of $n$ copies of $\mathbb{N}$. One main result is that when $G$ and the canonical shift map $\sigma$ admit a joint Følner sequence for their action on $\mathbb{N}$, then both group von Neumann algebras $L(G)$ and $L(H_n(G))$ are II${}_1$ factors with property Gamma and hence $G$ and $H_n(G)$ are inner amenable. More generally, let $\Gamma$ be an ICC group of permutations preserving the ends of $X_n$. We prove that if the natural near action on $\mathbb{N}$ of the germs of almost permutations arising from $\Gamma$ along some ray is amenable, then $L(\Gamma )$ has property Gamma; if this group of germs is itself amenable, then $L(\Gamma )$ is McDuff. Then we construct a shift-similar group $G$, using property (T) groups with certain actions, such that neither $G$ nor $H_n(G)$ is inner amenable. Finally, we produce extremely broad and diverse sources of shift-similar groups satisfying the joint amenability condition. These examples come from residually finite groups and from wobbling groups of a variety of metric spaces. In particular, we completely characterize all metrics $d$ on $\mathbb{N}$ for which the wobbling group $W(\mathbb{N},d)$ is shift-similar in terms of a coarse geometric property---namely, that ${\sigma }^{-1}$ is bornologous with respect to $d$. As a consequence, this gives us necessary and sufficient conditions for when the wobbling group of any countable metric space admits a shift-similar embedding.

arXiv comment69 pages, no figures. Comments welcome