We show an algorithm that for a given regular tree language L decides if L is in Pi^0_2, that is if L belongs to the second level of Borel Hierarchy. Moreover, if L is in Pi^0_2, then we construct a weak alternating automaton of index (0, 2) which recognises L. We also prove that for a given language L, L is recognisable by a weak alternating (1, 3)-automaton if and only if it is recognisable by a weak non-deterministic (1, 3)-automaton.
@InProceedings{cavallari_et_al:LIPIcs.MFCS.2017.56, author = {Cavallari, Filippo and Michalewski, Henryk and Skrzypczak, Michal}, title = {{A Characterisation of Pi^0\underline2 Regular Tree Languages}}, booktitle = {42nd International Symposium on Mathematical Foundations of Computer Science (MFCS 2017)}, pages = {56:1--56:14}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-046-0}, ISSN = {1868-8969}, year = {2017}, volume = {83}, editor = {Larsen, Kim G. and Bodlaender, Hans L. and Raskin, Jean-Francois}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.MFCS.2017.56}, URN = {urn:nbn:de:0030-drops-80683}, doi = {10.4230/LIPIcs.MFCS.2017.56}, annote = {Keywords: infinite trees, Rabin-Mostowski hierarchy, regular languages} }
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