A remark on fullness of some group measure space von Neumann algebras

Abstract

Recently C. Houdayer and Y. Isono have proved among other things that every biexact group $\Gamma$ has the property that for any non-singular strongly ergodic action $\Gamma\curvearrowright (X,\mu)$ on a standard measure space the group measure space von Neumann algebra $\Gamma\ltimes L^\infty(X)$ is full. In this note, we prove the same property for a wider class of groups, notably including $\mathrm{SL}(3,{\mathbb Z})$. We also prove that for any connected simple Lie group $G$ with finite center, any lattice $\Gamma\le G$, and any closed non-amenable subgroup $H\le G$, the non-singular action $\Gamma\curvearrowright G/H$ is strongly ergodic and the von Neumann factor $\Gamma\ltimes L^\infty(G/H)$ is full.