Abstract

Some rigidity results for II$_1$ factors arising from wreath products of property (T) groups

Ionut Chifan
University of Iowa

We show that any infinite collection $(\Gamma_n)_{n\in \mathbb N}$ of icc, hyperbolic, property (T) groups satisfies the following von Neumann algebraic \emph{infinite product rigidity} phenomenon. If $\Lambda$ is an arbitrary group such that $L(\oplus_{n\in \mathbb N} \G_n)\cong L(\Lambda)$ then there exists an infinite direct sum decomposition $\Lambda=(\oplus_{n \in \mathbb N} \Lambda_n )\oplus A$ with $A$ icc amenable such that, for all $n\in \mathbb N$, up to amplifications, we have $L(\Gamma_n) \cong L(\Lambda_n)$ and $L(\oplus_{k\geq n} \Gamma_k )\cong L((\oplus_{k\geq n} \Lambda_k) \oplus A)$. The result is sharp and complements the previous finite product rigidity property found in \cite{CdSS16}. Using this we provide an uncountable family of restricted wreath products $\G=\Sigma\wr \Delta$ of icc, property (T) groups $\Sigma$, $\Delta$ whose wreath product structure is recognizable, up to a normal amenable subgroup, from their von Neumann algebras $L(\Gamma)$. Along the way we highlight several applications of these results to the study of rigidity in the $\mathbb C^*$-algebra setting. This is based on a joint work with Bogdan Udrea.