$C^*-$crossed product of groupoid actions on categories

TL;DR

This paper explores the construction of crossed product $C^*$-algebras from groupoid actions on categories, identifying a regular subcategory and establishing an isomorphism with a crossed product algebra.

Contribution

It introduces the concept of a regular subcategory and a quasi action, linking the semi-direct product category to a $C^*$-algebraic crossed product.

Findings

Existence of a regular subcategory $H_r$ with $H_r imes_eta G = H imes_eta G$

Construction of a quasi action $eta$ on $C^*(H_r)$

Isomorphism $C^*(H_r imes_eta G) = C^*(H_r) imes_eta G$

Abstract

Suppose that $G$ is a groupoid acting on a small category $H$ in the sense of \cite[Definition 4]{NOT} and $H\times_\alpha G$ is the resulting semi-direct product category (as in \cite[Proposition 8]{NOT}). We show that there exists a subcategory $H_r \subseteq H$ satisfying some nice property called ``regularity'' such that $H_r \times_\alpha G = H\times_\alpha G$. Moreover, we show that there exists a so-called ``quasi action'' (see Definition \ref{quasi}) $\beta$ of $G$ on $C^*(H_r)$ (where $C^*(H_r)$ is the semigroupoid $C^*$-algebra as defined in \cite{EXE}) such that $C^*(H_r\times_\alpha G) = C^*(H_r)\times_\beta G$ (where the crossed product for $\beta$ is as defined in Definition \ref{cross}).