Some remarks on Reduced $C^{\ast }$-algebras of semigroup dynamical systems and product systems

We study the exactness of the reduced crossed product of a semigroup dynamical system and the reduced $C^{\ast }$-algebra of a product system. We show that for a semigroup dynamical system $(A,P,\alpha )$, under reasonable hypotheses (e.g., $P$ is abelian and finitely generated), the reduced crossed product $A{⋊}_{red}P$ is exact if and only if $A$ is exact. This strengthens our earlier result (\cite{Amir_Sundar-product-system}), where it was assumed that the action of $P$ on $A$ is by injective endomorphisms. We also compare the groupoid crossed product described in \cite{Amir_Sundar-product-system} and the Fell bundle constructed in \cite{Rennie_Sims} for a product system, and show that they are equivalent as Fell bundles.