We clarify the relationship between basic constructions of semi-abelian category theory and the theory of ideals and clots in universal algebra. To name a few results in this frame, which establish connections between hitherto separated subjects, 0-regularity in universal algebra corresponds to the requirement that regular epimorphisms are normal; we describe clots in categorical terms and show that ideals are images of clots under regular epimorphisms; we show that the relationship between internal precrossed modules and internal reflexive graphs extends the relationship between compatible reflexive binary relations and clots.
a b s t r a c tIn this paper we study Morita contexts for semigroups. We prove a Rees matrix cover connection between strongly Morita equivalent semigroups and investigate how the existence of a unitary Morita semigroup over a given semigroup is related to the existence of a 'good' Rees matrix cover of this semigroup.
A ubiquitous class of lattice ordered semigroups introduced by Bosbach in 1991, which we will call Bezout monoids, seems to be the appropriate structure for the study of divisibility in various classical rings like GCD domains (including UFD's), rings of low dimension (including semihereditary rings), as well as certain subdirect products of such rings and certain factors of such subdirect products. A Bezout monoid is a commutative monoid S with 0 such that under the natural partial order (for a, b ∈ S, a ≤ b ∈ S ⇐⇒ bS ⊆ aS), S is a distributive lattice, multiplication is distributive over both meets and joins, and for any x, y ∈ S, if d = x ∧ y and dx 1 = x, then there is a y 1 ∈ S with dy 1 = y and x 1 ∧ y 1 = 1. In the present paper, Bezout monoids are investigated by using filters and m-prime filters. We also prove analogues of the Pierce and the Grothendieck sheaf representations of rings for Bezout monoids. The question as to whether Bezout monoids describe divisibility in Bezout rings (rings whose finitely generated ideals are principal) is still open.
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