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The Cuboid Lemma and Mal’tsev Categories

Dedicated to George Janelidze on the occasion of his 60th birthday

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Abstract

We prove that a regular category ℂ is a Mal’tsev category if and only if a strong form of the denormalised 3 × 3 Lemma holds true in ℂ. In this version of the 3 × 3 Lemma, the vertical exact forks are replaced by pullbacks of regular epimorphisms along arbitrary morphisms. The shape of the diagram it determines suggests to call it the Cuboid Lemma. This new characterisation of regular categories that are Mal’tsev categories (= 2-permutable) is similar to the one previously obtained for Goursat categories (= 3-permutable). We also analyse the “relative” version of the Cuboid Lemma and extend our results to that context.

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Correspondence to Marino Gran.

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Gran, M., Rodelo, D. The Cuboid Lemma and Mal’tsev Categories. Appl Categor Struct 22, 805–816 (2014). https://doi.org/10.1007/s10485-013-9352-5

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  • DOI: https://doi.org/10.1007/s10485-013-9352-5

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