The Haagerup property for groups and for tracial von Neumann algebras in terms of invariant and mixing states

Abstract

The aim of the article is to provide two characterizations of the Haagerup property: one for locally compact, second countable groups, and the other one for finite von Neumann algebras. Both are expressed in terms of approximations of some non-ergodic invariant states by mixing ones for actions on unital $C^{\ast }$-algebras on the one hand, and for pairs of tracial von Neumann algebras by mixing binormal states on the other hand.

To be published in Expositiones Mathematicae. Characterizations in terms of binormal states of relative property H for pairs of tracial von Neumann algebras on the one hand, and of property T for II${}_1$ factors on the other hand have been added; 19 pages; typos corrected and compatibility with the journal adapted