We consider the logic MSO+U, which is monadic second-order logic extended with the unbounding quantifier. The unbounding quantifier is used to say that a property of finite sets holds for sets of arbitrarily large size. We prove that the logic is undecidable on infinite words, i.e. the MSO+U theory of (N,<) is undecidable. This settles an open problem about the logic, and improves a previous undecidability result, which used infinite trees and additional axioms from set theory.
@InProceedings{bojanczyk_et_al:LIPIcs.STACS.2016.21, author = {Bojanczyk, Mikolaj and Parys, Pawel and Torunczyk, Szymon}, title = {{The MSO+U Theory of (N,\langle) Is Undecidable}}, booktitle = {33rd Symposium on Theoretical Aspects of Computer Science (STACS 2016)}, pages = {21:1--21:8}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-001-9}, ISSN = {1868-8969}, year = {2016}, volume = {47}, editor = {Ollinger, Nicolas and Vollmer, Heribert}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.STACS.2016.21}, URN = {urn:nbn:de:0030-drops-57223}, doi = {10.4230/LIPIcs.STACS.2016.21}, annote = {Keywords: automata, logic, unbounding quantifier, bounds, undecidability} }
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