Group von Neumann algebras, inner amenability, and unit groups of continuous rings

TL;DR

This paper links group properties to the amenability of unit groups in certain continuous rings, showing non-inner amenable groups lead to non-amenable unit groups in associated operator rings.

Contribution

It establishes a connection between group inner amenability and the non-amenability of unit groups in rings of affiliated operators, providing new examples of non-amenable continuous rings.

Findings

Non-inner amenable groups imply non-amenable unit groups in affiliated operator rings.

Examples of non-discrete irreducible continuous rings with non-amenable unit groups.

Connections with Eymard-Greenleaf amenability of unitary group actions.

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 $\mathrm{II}_{1}$ factor on the associated space of projections of a fixed trace.