SEMIGROUPS WHOSE IDEMPOTENTS FORM A SUBSEMIGROUP

We prove that if the “type-II-construct” subsemigroup of a finite semigroup S is regular, then the “type-II” subsemigroup of S is computable (actually in this case, type-II and type-II-construct are equal). This, together with certain older results about pseudo-varieties of finite semigroups, leads to further results: (1) We get a new proof of Ash's theorem: If the idempotents in a finite semigroup S commute, then S divides a finite inverse semigroup. Equivalently: The pseudo-variety generated by the finite inverse semigroups consists of those finite semigroups whose idempotents commute. (2) We prove: If the idempotents of a finite semigroup S form a subsemigroup then S divides a finite orthodox semigroup. Equivalently: The pseudo-variety generated by the finite orthodox semigroups consists of those finite semigroups whose idempotents form a subsemigroup. (3) We prove: The union of all the subgroups of a semigroup S forms a subsemigroup if and only if 5 belongs to the pseudo-variety u[formula omitted] * G if and only if Sn belongs to u[formula omitted]. Here u[formula omitted] denotes the pseudo-variety of finite semigroups which are unions of groups. For these three classes of semigroups, type-II is equal to type-II construct. © 1990, Australian Mathematical Society. All rights reserved.