Finite proper covers in a class of finite semigroups with commuting idempotents

Semigroup Forum, 66 (2003), 433-454


Weakly left ample semigroups are a class of semigroups that are (2,1) -subalgebras of semigroups of partial transformations, where the unary operation takes a transformation α to the identity map in the domain of α . It is known that there is a class of proper weakly left ample semigroups whose structure is determined by unipotent monoids acting on semilattices or categories. In this paper we show that for every finite weakly left ample semigroup S , there is a finite proper weakly left ample semigroup $\hat{S}$ and an onto morphism from $\hat{S}$ to S which separates idempotents. In fact, $\hat{S}$ is actually a (2,1) -subalgebra of a symmetric inverse semigroup, that is, it is a left ample semigroup (formerly, left type A).