The relative rank of the endomorphism monoid of a finite $G$-set

Abstract

For a group $G$ acting on a set $X$, let $\text{End}_G(X)$ be the monoid of all $G$-equivariant transformations, or $G$-endomorphisms, of $X$, and let $\text{Aut}_G(X)$ be its group of units. After discussing few basic results in a general setting, we focus on the case when $G$ and $X$ are both finite in order to determine the smallest cardinality of a set $W \subseteq \text{End}_G(X)$ such that $W \cup \text{Aut}_G(X)$ generates $\text{End}_G(X)$; this is known in semigroup theory as the relative rank of $\text{End}_G(X)$ modulo $\text{Aut}_G(X)$.