Relative Property (T) Actions and Trivial Outer Automorphism Groups

Damien Gaboriau (1)

1 UMPA-ENSL - Unité de Mathématiques Pures et Appliquées

We show that every non-amenable free product of groups admits free ergodic probability measure preserving actions which have relative property (T) in the sense of S.~Popa \cite[Def. 4.1]{Pop06}. There are uncountably many such actions up to orbit equivalence and von Neumann equivalence, and they may be chosen to be conjugate to any prescribed action when restricted to the free factors. We exhibit also, for every non-amenable free product of groups, free ergodic probability measure preserving actions whose associated equivalence relation has trivial outer automorphisms group. This gives in particular the first examples of such actions for the free group on $2$ generators.

Keywords: Relative Property (T); Outer Automorphism Groups; free products; orbit equivalence; von Neumann Algebras

Domains: Mathematics [math]; Operator Algebras [math.OA]; Mathematics [math]; Group Theory [math.GR]

Damien Gaboriau. Relative Property (T) Actions and Trivial Outer Automorphism Groups. 2008. ⟨ensl-00269298v1⟩