Some subdirectly irreducible idempotent semigroups

Subdirectly irreducible idempotent semigroups were characterized in [3], and in that paper, their connection with the various equational classes of idempotent semigroups was discussed. All these results are in terms of identities, so that examples of subdirectly irreducibles in the equational classes are explicitly known only for small classes. It is easy to show from general considerations (see the last section of the present paper) that every proper equational subclass of the class of idempotent semigroups is generated (as an equational class) by one or two subdirectly irreducibles. In this paper we give an example of a subdirectly irreducible for each join irreducible equational class of idempotent semigroups, which generates the class. This list, together with known results, gives explicit examples of one or two finite subdirectly irreducibles which generate the various equational classes. Research supported by the National Research Council of Canada.     

Global determinism of idempotent semigroups

The power semigroup, or global, of a semigroup S is the set 𝒫(S) of all nonempty subsets of S equipped with the naturally defined multiplication. A class 𝒦 of semigroups is globally determined if any two members of 𝒦 with isomorphic globals are themselves isomorphic. The principal goal of this paper is to prove that the class of all idempotent semigroups is globally determined.     

On finitely generated idempotent semigroups

We give two short proofs of the well-known fact that every finitely generated idempotent semigroup is finite.     

Commutative Noetherian semigroups are finitely generated

We provide a short proof that a commutative semigroup is finitely generated if its lattice of congruences is Noetherian.     

Unification in varieties of idempotent semigroups

We have classified all varieties of idempotent semigroups with respect to the unification types of their defining sets of identities. With the exception of eight finitary unifying theories these are all of unification type zero. This yields countably many examples of theories of that type which are more “natural” than the first example constructed by Fages and Huet [9, 10].     

The joint of the pseudovarieties of idempotent semigroups and locally trivial semigroups

Communicated by J.-E. Pin     

The join of the pseudovarieties of idempotent semigroups and locally trivial semigroups

This article solves a problem proposed by Almeida: the computation of the join of two well-known pseudovarieties of semigroups, namely the pseudovariety of bands and the pseudovariety of locally trivial semigroups. We use a method developed by Almeida, based on the theory of implicit operations. This work was partly supported by PRC Mathématiques et Informatique and by ESPRITBRA WG 6317 ASMICS-2     

Note on the orderability of idempotent semigroups

In [3] , we gave a condition for the orderability of finite idempotent semigroups. Recently, in [5], T. Saitô gives a condition for the orderability of idempotent semigroups in the general case. As Corollary ( 4–13 of [5] ), he obtains an idempotent semigroup S is orderable if and only if every finite subsemigroup of S is orderable. The purpose of this note is to give a direct proof of this result.     

Subgroups of free idempotent generated regular semigroups

In [6] it is shown that the maximal subgroups of the free idempotent generated regular semigroup which is determined by the biordered set of a completely O-simple semigroup are free. In this note we shall extend this result to a wider class of semigroups.     

Injectives in equational classes of idempotent semigroups

In this paper injectives in the various equational classes of idempotent semigroups are discussed. The classes with enough injectives are determined, and the injectives described in every class.