# Amenability and paradoxicality in semigroups and C*-algebras

## TL;DR

This paper explores the concepts of amenability and paradoxicality in semigroups and their associated C*-algebras, providing new characterizations and examples that deepen understanding of their algebraic and analytical properties.

## Contribution

It introduces novel characterizations of amenability in inverse semigroups and their C*-algebras, including conditions involving F{}lner sequences, traces, and K-theory, along with new examples.

## Key findings

- Existence of semigroup with algebraically amenable ring but no F{}lner sequence
- Equivalence of several conditions characterizing non-paradoxical sets in inverse semigroups
- Quasidiagonality of certain C*-algebras implies semigroup amenability

## Abstract

We analyze the dichotomy amenable/paradoxical in the context of (discrete, countable, unital) semigroups and corresponding semigroup rings. We consider also F{\o}lner's type characterizations of amenability and give an example of a semigroup whose semigroup ring is algebraically amenable but has no F{\o}lner sequence. In the context of inverse semigroups $S$ we give a characterization of invariant measures on $S$ (in the sense of Day) in terms of two notions: $domain$ $measurability$ and $localization$. Given a unital representation of $S$ in terms of partial bijections on some set $X$ we define a natural generalization of the uniform Roe algebra of a group, which we denote by $\mathcal{R}_X$. We show that the following notions are then equivalent: (1) $X$ is domain measurable; (2) $X$ is not paradoxical; (3) $X$ satisfies the domain F{\o}lner condition; (4) there is an algebraically amenable dense *-subalgebra of $\mathcal{R}_X$; (5) $\mathcal{R}_X$ has an amenable trace; (6) $\mathcal{R}_X$ is not properly infinite and (7) $[0]\not=[1]$ in the $K_0$-group of $\mathcal{R}_X$. We also show that any tracial state on $\mathcal{R}_X$ is amenable. Moreover, taking into account the localization condition, we give several C*-algebraic characterizations of the amenability of $X$. Finally, we show that for a certain class of inverse semigroups, the quasidiagonality of $C_r^*\left(X\right)$ implies the amenability of $X$. The converse implication is false.