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We study the complexity of model checking formulas in first-order logic parameterized by the number of distinct variables in the formula. This problem, which is not known to be fixed-parameter tractable, resisted to be properly classified in the context of parameterized complexity. We show that it is complete for a newly-defined complexity class that we propose as an analog of the classical class PSPACE in parameterized complexity. We support this intuition by the following findings: First, the proposed class admits a definition in terms of alternating Turing machines in a similar way as PSPACE can be defined in terms of polynomial-time alternating machines. Second, we show that parameterized versions of other PSPACE-complete problems, like winning certain pebble games and finding restricted resolution refutations, are complete for this class.
@InProceedings{berkholz_et_al:LIPIcs.FSTTCS.2014.109,
author = {Berkholz, Christoph and Elberfeld, Michael},
title = {{Parameterized Complexity of Fixed Variable Logics}},
booktitle = {34th International Conference on Foundation of Software Technology and Theoretical Computer Science (FSTTCS 2014)},
pages = {109--120},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-939897-77-4},
ISSN = {1868-8969},
year = {2014},
volume = {29},
editor = {Raman, Venkatesh and Suresh, S. P.},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.FSTTCS.2014.109},
URN = {urn:nbn:de:0030-drops-48367},
doi = {10.4230/LIPIcs.FSTTCS.2014.109},
annote = {Keywords: Parameterized complexity, polynomial space, first-order logic, pebble games, regular resolutions}
}