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East, James; Azeef Muhammed, P. A.

A groupoid approach to regular \(\ast \)-semigroups. (English) Zbl 1531.20071

Adv. Math. 437, Article ID 109447, 104 p. (2024).
Summary: In this paper we develop a new groupoid-based structure theory for the class of regular \(\ast \)-semigroups. This class occupies something of a ‘sweet spot’ between the important classes of inverse and regular semigroups, and contains many natural examples. Some of the most significant families include the partition, Brauer and Temperley-Lieb monoids, among other diagram monoids. Our main result is that the category of regular \(\ast \)-semigroups is isomorphic to the category of so-called ‘chained projection groupoids’. Such a groupoid is in fact a triple \((P, \mathcal{G}, \varepsilon)\), where:
\(\bullet\)
\(P\) is a projection algebra (in the sense of Imaoka and Jones),
\(\bullet\)
\(\mathcal{G}\) is an ordered groupoid with object set \(P\), and
\(\bullet\)
\(\varepsilon : \mathcal{C} \to \mathcal{G}\) is a special functor, where \(\mathcal{C}\) is a certain natural ‘chain groupoid’ constructed from \(P\).
Roughly speaking: the groupoid \(\mathcal{G} = \mathcal{G}(S)\) remembers only the ‘easy’ products in a regular \(\ast \)-semigroup \(S\); the projection algebra \(P = P(S)\) remembers only the ‘conjugation action’ of the projections of \(S\); and the functor \(\varepsilon = \varepsilon(S)\) tells us how \(\mathcal{G}\) and \(P\) ‘fit together’ in order to recover the entire structure of \(S\). In this way, we obtain the first completely general structure theorem for regular \(\ast \)-semigroups. As a consequence of our main result, we give a new proof of the celebrated Ehresmann-Schein-Nambooripad Theorem, which establishes an isomorphism between the categories of inverse semigroups and inductive groupoids. Other applications will be given in future works. We consider several examples along the way, and pose a number of problems that we believe are worthy of further attention.

MSC:

20M10 General structure theory for semigroups
20M50 Connections of semigroups with homological algebra and category theory
18B40 Groupoids, semigroupoids, semigroups, groups (viewed as categories)
20M17 Regular semigroups
20M20 Semigroups of transformations, relations, partitions, etc.
20M05 Free semigroups, generators and relations, word problems

Keywords:

regular \(\ast \)-semigroups; inverse semigroups; groupoids; projection algebras; partition monoids

Software:

Semigroups; Prover9; Mace4; GAP
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References:

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