Minimal compact group actions on C$^*$-algebras with simple fixed point algebras

TL;DR

This paper proves that minimal actions of second countable compact groups on separable C*-algebras with simple fixed point algebras are quasi-product, extending known results beyond abelian and profinite groups, with applications to classification theory.

Contribution

It establishes that faithful minimal actions with simple fixed point algebras are quasi-product for a broad class of groups, using subfactor techniques and ergodic theory.

Findings

Such actions are quasi-product when fixed point algebra is simple.

If fixed point algebra is a Kirchberg algebra, the action is shift-absorbing and classifiable.

Extends previous results from abelian and profinite groups to all second countable compact groups.

Abstract

The notion of qausi-product actions of a compact group on a C$^*$-algebra was introduced by Bratteli et al. in their attempt to seek an equivariant analogue of Glimm's characterization of non-type I C$^*$-algebras. We show that a faithful minimal action of a second countable compact group on a separable C$^*$-algebra is quasi-product whenever its fixed point algebra is simple. This was previously known only for compact abelian groups and for profinite groups. Our proof relies on a subfactor technique applied to finite index inclusions of simple C$^*$-algebras in the purely infinite case, and also uses ergodic actions of compact groups in the general case. As an application, we show that if moreover the fixed point algebra is a Kirchberg algebra, such an action is always isometrically shift-absorbing, and hence is classifiable by the equivariant KK-theory due to a recent result of Gabe-Szab\'o.