We study the strength of axioms needed to prove various results related to automata on infinite words and Büchi's theorem on the decidability of the MSO theory of (N, less_or_equal). We prove that the following are equivalent over the weak second-order arithmetic theory RCA: 1. Büchi's complementation theorem for nondeterministic automata on infinite words, 2. the decidability of the depth-n fragment of the MSO theory of (N, less_or_equal), for each n greater than 5, 3. the induction scheme for Sigma^0_2 formulae of arithmetic. Moreover, each of (1)-(3) is equivalent to the additive version of Ramsey's Theorem for pairs, often used in proofs of (1); each of (1)-(3) implies McNaughton's determinisation theorem for automata on infinite words; and each of (1)-(3) implies the "bounded-width" version of König's Lemma, often used in proofs of McNaughton's theorem.
@InProceedings{kolodziejczyk_et_al:LIPIcs.CSL.2016.36, author = {Kolodziejczyk, Leszek Aleksander and Michalewski, Henryk and Pradic, C\'{e}cilia and Skrzypczak, Michal}, title = {{The Logical Strength of B\"{u}chi's Decidability Theorem}}, booktitle = {25th EACSL Annual Conference on Computer Science Logic (CSL 2016)}, pages = {36:1--36:16}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-022-4}, ISSN = {1868-8969}, year = {2016}, volume = {62}, editor = {Talbot, Jean-Marc and Regnier, Laurent}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CSL.2016.36}, URN = {urn:nbn:de:0030-drops-65765}, doi = {10.4230/LIPIcs.CSL.2016.36}, annote = {Keywords: nondeterministic automata, monadic second-order logic, B\"{u}chi's theorem, additive Ramsey's theorem, reverse mathematics} }
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