Abstract
A split extension of monoids with kernel k :N rightarrow G, cokernel e :G rightarrow H and splitting s :H rightarrow G is weakly Schreier if each element g in G can be written g = k(n)se(g) for some n in N. The characterization of weakly Schreier extensions allows them to be viewed as something akin to a weak semidirect product. The motivating examples of such extensions are the Artin glueings of topological spaces and, of course, the Schreier extensions of monoids which they generalise. In this paper we show that the lambda -semidirect products of inverse monoids are also examples of weakly Schreier extensions. The characterization of weakly Schreier extensions sheds some light on the structure of lambda -semidirect products. The set of weakly Schreier extensions between two monoids comes equipped with a natural poset structure, which induces an order on the set of lambda -semidirect products between two inverse monoids. We show that Artin glueings are in fact lambda -semidirect products and inspired by this identify a class of Artin-like lambda -semidirect products. We show that joins exist for this special class of lambda -semidirect product in the aforementioned order.
Highlights
The ideas underlying the semidirect product of groups can be adapted to a number of structures. One such example is the context of semigroups wherein an action of semigroups α : H × N → N gives rise to a semidirect product N α H defined just as in the group case
Given inverse semigroups N and H the idea is to use an action of H on N to equip a certain subset of N × H with a multiplication turning it into an inverse semigroup
We outline the basics of inverse semigroups and Billhardt’s λ-semidirect products [1] before discussing frames and Artin glueings
Summary
The ideas underlying the semidirect product of groups can be adapted to a number of structures One such example is the context of semigroups wherein an action of semigroups α : H × N → N gives rise to a semidirect product N α H defined just as in the group case. These semidirect products have found much use in semigroup theory, for instance they provide some insight into the structure of inverse semigroups. We show that this class is closed under binary joins
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