Group von Neumann Algebras, Inner Amenability, and Unit Groups of Continuous Rings

Abstract

Abstract We prove that, if a discrete group $G$ is not inner amenable, then the unit group of the ring of operators affiliated with the group von Neumann algebra of $G$ is non-amenable with respect to the topology generated by its rank metric. This provides examples of non-discrete irreducible, continuous rings (in von Neumann’s sense) whose unit groups are non-amenable with regard to the rank topology. Our argument establishes and uses connections with Eymard–Greenleaf amenability of the action of the unitary group of a $\textrm{II}_{1}$ factor on the associated space of projections of a fixed trace.