Percorrer por autor "Hoefnagel, Michael"
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- Categorical aspects of congruence distributivityPublication . Hoefnagel, Michael; Rodelo, DianaAbstract. We study a categorical condition on relations, which is a categorical formulation of Jónsson’s characterisation of congruence distributive varieties. Categories satisfying these conditions need not be varieties; for instance, the dual of the categories of topological spaces, ordered sets, G-sets, and the dual of any (pre)topos all provide us with examples.
- On difunctionality of class relationsPublication . Hoefnagel, Michael; Janelidze, Zurab; Rodelo, DianaFor a given variety V of algebras, we define a class relation to be a binary relation R subset of S(2)which is of the form R = S-2 boolean AND K for some congruence class K on A(2), where A is an algebra in V such that S subset of A. In this paper we study the following property of V : every reflexive class relation is an equivalence relation. In particular, we obtain equivalent characterizations of this property analogous to well-known equivalent characterizations of congruence-permutable varieties. This property determines a Mal'tsev condition on the variety and in a suitable sense, it is a join of Chajda's egg-box property as well as Duda's direct decomposability of congruence classes.