Globals of Pseudovarieties of Commutative Semigroups: the Finite Basis Problem, Decidability, and Gaps

Decidability and Tameness in the Theory of Finite Semigroups

The globals of pseudovarieties of ordered semigroups containing B2 and an application to a problem proposed by Pin

Given a basis of pseudoidentities for a pseudovariety of ordered semigroups containing the 5-element aperiodic Brandt semigroup B2, under the natural order, it is shown that the same basis, over the most general graph over which it can be read, defines the global. This is used to show that the global of the level 3/2 of the refinement of Straubing-Thérien’s concatenation hierarchy introduced by...

Tameness of The Pseudovariety Ls1

A class of finite semigroups V is said to be decidable if the membership problem for V has a solution, that is, if we can construct an algorithm to test whether a given semigroup lies in V. Decidability of pseudovarieties is not preserved by some of the most common pseudovariety operators, such as semidirect product, Mal’cev product and join [1, 17]. In particular Rhodes [17] has exhibited a de...

On the Irreducibility of Pseudovarieties of Semigroups

We show that, for every pseudovariety of groups H, the pseudovariety H̄, consisting of all finite semigroups all of whose subgroups lie in H, is irreducible for join and the Mal’cev and semidirect products. The proof involves a Rees matrix construction which motivates the study of iterated Mal’cev products with the pseudovariety of bands. We further provide a strict infinite filtration for H̄ usi...

Tameness of pseudovariety joins

The concept of tameness of a pseudovariety was introduced by Almeida and Steinberg [1] as a tool for proving decidability of the membership problem for semidirect products of pseudovarieties. Recall that the join V ∨ W of two pseudovarieties V and W is the least pseudovariety containing both V and W. This talk is concerned with the problem of proving tameness of joins. This problem was consider...