Abstract

Mal’tsev categories turned out to be a central concept in categorical algebra. On the one hand, the simplicity and the beauty of the notion is revealed by the wide variety of characterizations of a markedly different flavour. Depending on the context, one can define Mal’tsev categories as those for which ‘any reflexive relation is an equivalence’; ‘any relation is difunctional’; ‘the composition of equivalence relations on the same object is commutative’; ‘each fibre of the fibration of points is unital’ or ‘the forgetful functor from internal groupoids to reflexive graphs is saturated on subobjects’. For a variety of universal algebras, these are also equivalent to the existence in its algebraic theory of a Mal’tsev operation, i.e. a ternary operation p satisfying the axioms p = y and p = x. On the other hand, Mal’tsev categories have been shown to be the right context in which to develop the theory of centrality of equivalence relations, Baer sums of extensions, and some homological lemmas such as the denormalized 3 × 3 Lemma, whose validity in a regular category is equivalent to the weaker ‘Goursat property’, which has also turned out to be of wide interest.