Reduced $C^{*}$-algebras of Product Systems -- an $E_0$-semigroup and a Groupoid perspective

TL;DR

This paper establishes a correspondence between $E_0$-semigroups and product systems over Ore semigroups, and analyzes the reduced $C^{*}$-algebras' properties using groupoid techniques, linking nuclearity, exactness, and $K$-theory invariance.

Contribution

It introduces a bijection between $E_0$-semigroups and product systems over Ore semigroups and connects their reduced $C^{*}$-algebras to crossed products via groupoid methods.

Findings

Reduced $C^{*}$-algebra of a proper product system is Morita equivalent to the crossed product.

Nuclearity and exactness of the reduced $C^{*}$-algebra are characterized by the coefficient algebra.

$K$-theory is invariant under homotopy of product systems.

Abstract

For Ore semigroups $P$ with an order unit, we prove that there is a bijection between $E_0$-semigroups over $P$ and product systems of $C^{*}$-correspondences over $P^{op}$. We exploit this bijection and show that the reduced $C^{*}$-algebra of a proper product system is Morita equivalent to the reduced crossed product of the associated semigroup dynamical system given by the corresponding $E_0$-semigroup. We appeal to the groupoid picture of the reduced crossed product of a semigroup dynamical system derived in [47] to prove that, under good conditions, the reduced $C^{*}$-algebra of a proper product system is nuclear/exact if and only if the coefficient algebra is nuclear/exact. We also discuss the invariance of $K$-theory under homotopy of product systems.