English

Ideally regular categories

Category Theory 2026-03-03 v2

Abstract

In this note, we propose a generalisation of G. Janelidze's notion of an ideally exact category beyond the Barr exact setting. We define an ideally regular category as a regular, Bourn protomodular category with finite coproducts in which the unique morphism 0 -> 1 is effective for descent. As in the ideally exact case, ideally regular categories support a notion of ideal that classifies regular quotients. Moreover, they admit a characterisation in terms of monadicity over a homological category (rather than a semi-abelian one, as in the exact setting). Examples include Bourn protomodular quasivarieties of universal algebra in which 0 -> 1 is effective for descent (such as the category of torsion-free unital rings), all Bourn protomodular topological varieties with at least one constant (such as topological rings), and all semi-localisations of ideally exact categories.

Keywords

Cite

@article{arxiv.2511.21913,
  title  = {Ideally regular categories},
  author = {Sandra Mantovani and Mariano Messora},
  journal= {arXiv preprint arXiv:2511.21913},
  year   = {2026}
}

Comments

13 pages. Revised and published in Theory and Applications of Categories (at http://www.tac.mta.ca/tac/volumes/45/11/45-11abs.html )