Finitary conditions in special semidirect products

Abstract: We initiate the study of finitary conditions for monoids arising as expansions via the semidirect product construction of a monoid $M$ acting on the left of its power set (which we term `special semidirect products', and denote by $\mathcal{S}(M)$). Specifically, we examine the condition of weak left coherence (every finitely generated left ideal has a finite presentation as a left act); the related conditions of property (L), left ideal Howson, finitely left equated; and the left-right dual properties of weak right coherence, property (R), right ideal Howson and finitely right equated. Each of these conditions is preserved under retract, from which it is immediate that if $\mathcal{S}(M)$ satisfies one of our finitary conditions, then so must $M$, but the converse is not true. Indeed, we show that $\mathcal{S}(M)$ satisfies property (L) (or property (R)) if and only if $M$ is finite. We provide a characterisation of the monoids $M$ such that $\mathcal{S}(M)$ is left (respectively, right) ideal Howson, and give sufficient conditions for $\mathcal{S}(M)$ to be finitely left (respectively, right) equated and hence weakly left (respectively, right) coherent.