Ideals and clots in universal algebra and in semi-abelian categories

Abstract

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.