The powerset monad on the category of sets does not distribute over itself. Nevertheless a weaker form of distributive law of the powerset monad over itself exists and it essentially stems from the canonical Egli-Milner extension of the powerset to the category of relations. On the other hand, any regular category yields a category of relations, and some regular categories also possess a powerset-like monad, as is the Vietoris monad on compact Hausdorff spaces. We derive the Egli-Milner extension in three different frameworks : sets, toposes, and compact Hausdorff spaces. We prove that it corresponds to a monotone weak distributive law in each case by showing that the multiplication extends to relations but the unit does not. We provide an application to coalgebraic determinization of alternating automata.
@InProceedings{goy_et_al:LIPIcs.ICALP.2021.132, author = {Goy, Alexandre and Petri\c{s}an, Daniela and Aiguier, Marc}, title = {{Powerset-Like Monads Weakly Distribute over Themselves in Toposes and Compact Hausdorff Spaces}}, booktitle = {48th International Colloquium on Automata, Languages, and Programming (ICALP 2021)}, pages = {132:1--132:14}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-195-5}, ISSN = {1868-8969}, year = {2021}, volume = {198}, editor = {Bansal, Nikhil and Merelli, Emanuela and Worrell, James}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ICALP.2021.132}, URN = {urn:nbn:de:0030-drops-142016}, doi = {10.4230/LIPIcs.ICALP.2021.132}, annote = {Keywords: Egli-Milner relation, weak extension, weak distributive law, weak lifting, powerset monad, Vietoris monad, topos, alternating automaton, generalized determinization} }
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