Spectral gap and strict outerness for actions of locally compact groups on full factors

Abstract

We prove that an outer action of a locally compact group $G$ on a full factor $M$ is automatically strictly outer, meaning that the relative commutant of $M$ in the crossed product is trivial. If moreover the image of $G$ in the outer automorphism group $\mathrm{Out}M$ is closed, we prove that the crossed product remains full. We obtain this result by proving that the inclusion of $M$ in the crossed product automatically has a spectral gap property. Such results had only been proven for actions of discrete groups and for actions of compact groups, by using quite different methods in both cases. Even for the canonical Bogoljubov actions on free group factors or free Araki-Woods factors, these results are new.

v2: minor changes, final version, to appear in Enseignement Mathematique, special issue in honor of Vaughan Jones

Cited by in corpus (2)

• Ergodic states on type III${}_1$ factors and ergodic actions
• Strictly outer actions of locally compact groups: beyond the full factor case