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A new Galois structure in the category of internal preorders. (English) Zbl 1435.18009

Summary: Let \(\mathsf{PreOrd}(\mathbb{C})\) be the category of internal preorders in an exact category \(\mathbb{C}\). We show that the pair (\(\mathsf{Eq}(\mathbb{C}),\,\mathsf{ParOrd}(\mathbb{C})\)) is a pretorsion theory in \(\mathsf{PreOrd}(\mathbb{C})\), where \(\mathsf{Eq}(\mathbb{C})\) and \(\mathsf{ParOrd}(\mathbb{C})\) are the full subcategories of internal equivalence relations and of internal partial orders in \(\mathbb{C}\), respectively. We observe that \(\mathsf{ParOrd}(\mathbb{C})\) is a reflective subcategory of \(\mathsf{PreOrd}(\mathbb{C})\) such that each component of the unit of the adjunction is a pullback-stable regular epimorphism. The reflector \(F\): \(\mathsf{PreOrd}(\mathbb{C}) \to \mathsf{ParOrd}(\mathbb{C})\) turns out to have stable units in the sense of Cassidy, Hébert and Kelly, thus inducing an admissible categorical Galois structure. In particular, when \(\mathbb{C}\) is the category \(\mathsf{Set}\) of sets, we show that this reflection induces a monotone-light factorization system (in the sense of [A. Carboni et al., Appl. Categ. Struct. 5, No. 1, 1–58 (1997; Zbl 0866.18003)]) in \(\mathsf{PreOrd(Set)}\). A topological interpretation of our results in the category of Alexandroff-discrete spaces is also given, via the well-known isomorphism between this latter category and \(\mathsf{PreOrd(Set)}\).

MSC:

18E50 Categorical Galois theory
18A32 Factorization systems, substructures, quotient structures, congruences, amalgams
18B35 Preorders, orders, domains and lattices (viewed as categories)
18E40 Torsion theories, radicals
06A15 Galois correspondences, closure operators (in relation to ordered sets)

Citations:

Zbl 0866.18003

References:

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[21] J. Xarez, The monotone-light factorization for categories via preordered and ordered sets, PhD Thesis (2003), University of Aveiro. This article may be accessed at http://www.tac.mta.ca/tac/ THEORY AND APPLICATIONS OF CATEGORIES will disseminate articles that significantly advance the study of categorical algebra or methods, or that make significant new contributions to mathematical science using categorical methods. The scope of the journal includes: all areas of pure category theory, including higher dimensional categories; applications of category theory to algebra, geometry and topology and other areas of mathematics; applications of category theory to computer science, physics and other mathematical sciences; contributions to scientific knowledge that make use of categorical methods. Articles appearing in the journal have been carefully and critically refereed under the responsibility of members of the Editorial Board. Only papers judged to be both significant and excellent are accepted for publication. Subscription information Individual subscribers receive abstracts of articles by e-mail as they are published. To subscribe, send e-mail to tac@mta.ca including a full name and postal address. Full text of the journal is freely available at http://www.tac.mta.ca/tac/.
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[24] Clemens Berger, Université de Nice-Sophia Antipolis: cberger@math.unice.fr Julie Bergner, University of Virginia: jeb2md (at) virginia.edu Richard Blute, Université d’ Ottawa: rblute@uottawa.ca
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[28] Joachim Kock, Universitat Autònoma de Barcelona: kock (at) mat.uab.cat Stephen Lack, Macquarie University: steve.lack@mq.edu.au F. William Lawvere, State University of New York at Buffalo: wlawvere@buffalo.edu Tom Leinster, University of Edinburgh: Tom.Leinster@ed.ac.uk
[29] Matias Menni, Conicet and Universidad Nacional de La Plata, Argentina: matias.menni@gmail.com Ieke Moerdijk, Utrecht University: i.moerdijk@uu.nl Susan Niefield, Union College: niefiels@union.edu Robert Paré, Dalhousie University: pare@mathstat.dal.ca Kate Ponto, University of Kentucky: kate.ponto (at) uky.edu Robert Rosebrugh, Mount Allison University: rrosebrugh@mta.ca Jiri Rosicky, Masaryk University: rosicky@math.muni.cz Giuseppe Rosolini, Università di Genova: rosolini@disi.unige.it Alex Simpson, University of Ljubljana: Alex.Simpson@fmf.uni-lj.si James Stasheff, University of North Carolina: jds@math.upenn.edu Ross Street, Macquarie University: ross.street@mq.edu.au Tim Van der Linden, Université catholique de Louvain: tim.vanderlinden@uclouvain.be
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