Completely injective semigroups with central idempotents
Edmund H. Feller, R. L. Gantos · Glasgow Mathematical Journal · 1969
A right [left] unitary S-system is a set M with right [left] operators in a semigroup S with 1, where x1 = x [1x = x] for all x ∈ M. We define a semigroup S with 1 to be completely right [left] injective provided that every right [left] unitary S-system is injective. The main purpose of this paper is to determine a structure for completely right [left] injective semigroups whose idempotents are in the centre.