We argue that cartesian bicategories, often used as a general categorical algebra of relations, are also a natural setting for the study of the axiom of choice (AC). In this setting, AC manifests itself as an inequation asserting that every total relation contains a map. The generality of cartesian bicategories allows us to separate this formulation from other set-theoretically equivalent properties, for instance that epimorphisms split. Moreover, via a classification result, we show that cartesian bicategories satisfying choice tend to be those that arise from bicategories of spans.
@InProceedings{bonchi_et_al:LIPIcs.CALCO.2019.15, author = {Bonchi, Filippo and Seeber, Jens and Soboci\'{n}ski, Pawe{\l}}, title = {{The Axiom of Choice in Cartesian Bicategories}}, booktitle = {8th Conference on Algebra and Coalgebra in Computer Science (CALCO 2019)}, pages = {15:1--15:17}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-120-7}, ISSN = {1868-8969}, year = {2019}, volume = {139}, editor = {Roggenbach, Markus and Sokolova, Ana}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CALCO.2019.15}, URN = {urn:nbn:de:0030-drops-114439}, doi = {10.4230/LIPIcs.CALCO.2019.15}, annote = {Keywords: Cartesian bicategories, Axiom of choice, string diagrams} }
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