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We study a new extension of the weak MSO logic, talking about boundedness. Instead of a previously considered quantifier 𝖴, expressing the fact that there exist arbitrarily large finite sets satisfying a given property, we consider a generalized quantifier 𝖴, expressing the fact that there exist tuples of arbitrarily large finite sets satisfying a given property. First, we prove that the new logic WMSO+𝖴_{tup} is strictly more expressive than WMSO+𝖴. In particular, WMSO+𝖴_{tup} is able to express the so-called simultaneous unboundedness property, for which we prove that it is not expressible in WMSO+𝖴. Second, we prove that it is decidable whether the tree generated by a given higher-order recursion scheme satisfies a given sentence of WMSO+𝖴_{tup}.
@InProceedings{badyl_et_al:LIPIcs.CSL.2024.12,
author = {Badyl, Anita and Parys, Pawe{\l}},
title = {{Extending the WMSO+U Logic with Quantification over Tuples}},
booktitle = {32nd EACSL Annual Conference on Computer Science Logic (CSL 2024)},
pages = {12:1--12:20},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-310-2},
ISSN = {1868-8969},
year = {2024},
volume = {288},
editor = {Murano, Aniello and Silva, Alexandra},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CSL.2024.12},
URN = {urn:nbn:de:0030-drops-196557},
doi = {10.4230/LIPIcs.CSL.2024.12},
annote = {Keywords: Boundedness, logic, decidability, expressivity, recursion schemes}
}