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Normality for monoid monomorphisms. (English) Zbl 1531.18003

A clot, a positive cone and a normal subobject of a monoid are defined as the zero-class of an internal reflexive relation, of a preorder or of a congruence, respectively. For a category \(\mathcal R\), a normal monomorphism with respect to \(\mathcal R\) as well as the normalization functor \(N\colon {\mathcal R} \to \) Mono(C), where C is a certain category, are defined. It is proved that if C is a pointed, well-povered and finitely complete category with arbitrary intersections and \(\mathcal R\) a pullback stable full subcategory of the one of internal relations on C closed under arbitrary intersections, then the normalization functor \(N\colon {\mathcal R} \to \) Mono(C) has a left adjoint \(F\colon\) Mono(C)\(\to \mathcal R\). Several equivalent conditions for a monoid monomorphism \(m\colon M\to A\) to be a normal subobject (a positive cone, a clot, respectively) are derived (here \(m(M)\) is identified with \(M\)). For example, a monoid monomorphism \(m\colon M\to A\) is a normal subobject (a positive cone, a clot, respectively) if and only if for every \(x,y\in A\) and every \(u\in M\), \(xy\in M \iff xuy\in M\) (\(xy\in M \Rightarrow xuy\in M; xy=1\Rightarrow xuy\in M\), respectively). Eleven examples were constructed to illustrate different aspects of the topics covered.

MSC:

18A22 Special properties of functors (faithful, full, etc.)
20M50 Connections of semigroups with homological algebra and category theory
20M20 Semigroups of transformations, relations, partitions, etc.
20M99 Semigroups
06F05 Ordered semigroups and monoids
18A40 Adjoint functors (universal constructions, reflective subcategories, Kan extensions, etc.)
18A20 Epimorphisms, monomorphisms, special classes of morphisms, null morphisms

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