Equivariant (co)module nuclearity of $C^*$-crossed products

Abstract

We define an equivariant and equicovariant versions of the notion of module nuclearity. More precisely, for a discrete group $\Gamma$ and operator $\mathcal A$-$\Gamma$-(co)module $\mathcal B$, $\mathcal E$ over a $\Gamma$-C$^*$-algebra $\mathcal A$, we define $\mathcal E$-$\Gamma$-nuclearity of $\mathcal B$, as an equivariant version of the notion of $\mathcal E$-nuclearity, in which the identity map on $\mathcal B$ is required to be approximately factored through matrix algebras on $\mathcal E$ with module structures coming both from the original module structure of $\mathcal E$ and the $\Gamma$-action on $\mathcal E$. For trivial actions of $\Gamma$, this is shown to reduce to the notion of module nuclearity, introduced and studied by the first author. As a concrete example, for a discrete group $\Gamma$ acting amenably on a unital C$^*$-algebra $\mathcal A$, we show that the reduced crossed product $\mathcal A\rtimes_{r} \Gamma$ is $\mathcal A$-$\Gamma$-nuclear. Conversely, if $\mathcal A$ is a nuclear C$^*$-algebra with a $\Gamma$-invariant state $\rho$ and $\mathcal A\rtimes_{r} \Gamma$ is $\mathcal A$-$\Gamma$-nuclear, then we deduce that $\Gamma$ is amenable. We show that when $\mathcal A\rtimes_{r} \Gamma$ is $\mathcal A$-$\Gamma$-nuclear and $\mathcal A$ has the completely bounded approximation property (resp., is exact), then so is $\mathcal A\rtimes_{r} \Gamma$. We prove similar results for $\mathcal A\rtimes_{r} \Gamma$, regarded as an $\mathcal A$-$\Gamma$-comodule.