We study the topological complexity of languages of Büchi automata on infinite binary trees. We show that such a language is either Borel and WMSO-definable, or Sigma_1^1-complete and not WMSO-definable; moreover it can be algorithmically decided which of the two cases holds. The proof relies on a direct reduction to deciding the winner in a finite game with a regular winning condition.
@InProceedings{skrzypczak_et_al:LIPIcs.ICALP.2016.99, author = {Skrzypczak, Michal and Walukiewicz, Igor}, title = {{Deciding the Topological Complexity of B\"{u}chi Languages}}, booktitle = {43rd International Colloquium on Automata, Languages, and Programming (ICALP 2016)}, pages = {99:1--99:13}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-013-2}, ISSN = {1868-8969}, year = {2016}, volume = {55}, editor = {Chatzigiannakis, Ioannis and Mitzenmacher, Michael and Rabani, Yuval and Sangiorgi, Davide}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ICALP.2016.99}, URN = {urn:nbn:de:0030-drops-62346}, doi = {10.4230/LIPIcs.ICALP.2016.99}, annote = {Keywords: tree automata, non-determinism, Borel sets, topological complexity, decidability} }
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