A Dixmier type averaging property of automorphisms on a C∗-algebra

Abstract

In this study of the relative Dixmier property for inclusions of von Neumann algebras and of [Formula: see text]-algebras, Popa considered a certain property of automorphisms on [Formula: see text]-algebras, that we here call the strong averaging property. In this paper, we characterize when an automorphism on a [Formula: see text]-algebra has the strong averaging property. In particular, automorphisms on commutative [Formula: see text]-algebras possess this property precisely when they are free. An automorphism on a unital separable simple [Formula: see text]-algebra with at least one tracial state has the strong averaging property precisely when its extension to the finite part of the bi-dual of the [Formula: see text]-algebra is properly outer, and in the simple, non-tracial case the strong averaging property is equivalent to being outer. To illustrate the usefulness of the strong averaging property we give three examples where we can provide simpler proofs of existing results on crossed product [Formula: see text]-algebras, and we are also able to extend these results in different directions.