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We show that the separation property fails for the classes Sigma_n of the Rabin-Mostowski index hierarchy of alternating automata on infinite trees. This extends our previous result (obtained with Szczepan Hummel) on the failure of the separation property for the class Sigma_2 (i.e., for co-Buchi sets). It remains open whether the separation property does hold for the classes Pi_n of the index hierarchy. To prove our result, we first consider the Rabin-Mostowski index hierarchy of deterministic automata on infinite words, for which we give a complete answer (generalizing previous results of Selivanov): the separation property holds for Pi_n and fails for Sigma_n-classes. The construction invented for words turns out to be useful for trees via a suitable game.
@InProceedings{arnold_et_al:LIPIcs.STACS.2012.396,
author = {Arnold, Andr\'{e} and Michalewski, Henryk and Niwinski, Damian},
title = {{On the separation question for tree languages}},
booktitle = {29th International Symposium on Theoretical Aspects of Computer Science (STACS 2012)},
pages = {396--407},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-939897-35-4},
ISSN = {1868-8969},
year = {2012},
volume = {14},
editor = {D\"{u}rr, Christoph and Wilke, Thomas},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.STACS.2012.396},
URN = {urn:nbn:de:0030-drops-34156},
doi = {10.4230/LIPIcs.STACS.2012.396},
annote = {Keywords: Alternating automata on infinite trees, Index hierarchy, Separation property}
}