We prove that relational structures admitting specific polymorphisms (namely, canonical pseudo-WNU operations of all arities n ≥ 3) have low relational width. This implies a collapse of the bounded width hierarchy for numerous classes of infinite-domain CSPs studied in the literature. Moreover, we obtain a characterization of bounded width for first-order reducts of unary structures and a characterization of MMSNP sentences that are equivalent to a Datalog program, answering a question posed by Bienvenu et al.. In particular, the bounded width hierarchy collapses in those cases as well.
@InProceedings{mottet_et_al:LIPIcs.ICALP.2021.138, author = {Mottet, Antoine and Nagy, Tom\'{a}\v{s} and Pinsker, Michael and Wrona, Micha{\l}}, title = {{Smooth Approximations and Relational Width Collapses}}, booktitle = {48th International Colloquium on Automata, Languages, and Programming (ICALP 2021)}, pages = {138:1--138:20}, 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.2021.138}, URN = {urn:nbn:de:0030-drops-142075}, doi = {10.4230/LIPIcs.ICALP.2021.138}, annote = {Keywords: local consistency, bounded width, constraint satisfaction problems, polymorphisms, smooth approximations} }
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