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Research Project

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A criterion for reflectiveness of normal extensionsExpand Expand

Publication . Montoli, Andrea; Rodelo, Diana; Van der Linden, Tim

We give a new sufficient condition for the normal extensions in an admissible Galois structure to be reflective. We then show that this condition is indeed fulfilled when X is the (protomodular) reflective subcategory of S-special objects of a Barr-exact S-protomodular category C, where S is the class of split epimorphic trivial extensions in C. Next to some concrete examples where the criterion may be applied, we also study the adjunction between a Barr-exact unital category and its abelian core, which we prove to be admissible.

Variations of the Shifting Lemma and Goursat categoriesExpand Expand

Publication . Gran, Marino; Rodelo, Diana; Nguefeu, Idriss Tchoffo

We prove that Mal'tsev and Goursat categories may be characterized through variations of the Shifting Lemma, that is classically expressed in terms of three congruences R, S and T, and characterizes congruence modular varieties. We first show that a regular category C is a Mal'tsev category if and only if the Shifting Lemma holds for reflexive relations on the same object in C. Moreover, we prove that a regular category C is a Goursat category if and only if the Shifting Lemma holds for a reflexive relation S and reflexive and positive relations R and T in C. In particular this provides a new characterization of 2-permutable and 3-permutable varieties and quasi-varieties of universal algebras.

Some remarks on connectors and groupoids in goursat categoriesExpand Expand

Publication . Gran, Marino; Nguefeu, Idriss Tchoffo; Rodelo, Diana

We prove that connectors are stable under quotients in any (regular) Goursat category. As a consequence, the category Conn(C) of connectors in C is a Goursat category whenever C is. This implies that Goursat categories can be characterised in terms of a simple property of internal groupoids.

Beck-Chevalley condition and Goursat categoriesExpand Expand

Publication . Gran, Marino; Rodelo, Diana

We characterise regular Goursat categories through a specific stability property of regular epimorphisms with respect to pullbacks. Under the assumption of the existence of some pushouts this property can be also expressed as a restricted Beck Chevalley condition, with respect to the fibration of points, for a special class of commutative squares. In the case of varieties of universal algebras these results give, in particular, a structural explanation of the existence of the ternary operations characterising 3-permutable varieties of universal algebras.

Stability properties characterising n- permutable categoriesExpand Expand

Publication . Jacqmin, Pierre-Alain; Rodelo, Diana

The purpose of this paper is two-fold. A first and more concrete aim is to characterise n-permutable categories through certain stability properties of regular epimorphisms. These characterisations allow us to recover the ternary terms and the (n + 1)-ary terms describing n-permutable varieties of universal algebras. A second and more abstract aim is to explain two proof techniques, by using the above characterisation as an opportunity to provide explicit new examples of their use: an embedding theorem for n-permutable categories which allows us to follow the varietal proof to show that an n-permutable category has certain properties; the theory of unconditional exactness properties which allows us to remove the assumption of the existence of colimits, in particular when we use the approximate co-operations approach to show that a regular category is n-permutable.

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## Funding agency

Fundação para a Ciência e a Tecnologia

## Funding programme

5876

## Funding Award Number

UID/MAT/00324/2013