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[Schaefer and Stefankovic, Theory of Computing Systems, 2015] provided an explicit formulation of EXISTS-R as the class capturing the complexity of deciding the Existential Theory of the Reals, and established that deciding, given a 3-player game, whether or not it has a Nash equilibrium with no probability exceeding a given rational is EXISTS-R-complete. Four more decision problems about Nash equilibria for 3-player games were very recently shown EXISTS-R-complete via a chain of individual, problem-specific reductions in [Garg et al., Proceedings of ICALP 2015]; determining more such EXISTS-R-complete problems was posed there as an open problem. In this work, we deliver an extensive catalog of EXISTS-R-complete decision problems about Nash equilibria in 3-player games, thus resolving completely the open problem from [Garg et al., Proceedings of ICALP 2015]. Towards this end, we present a single and very simple, unifying reduction from the EXISTS-R-complete decision problem from [Schaefer and Stefankovic, Theory of Computing Systems, 2015] to (almost) all the decision problems about Nash equilibria that were before shown NP-complete for 2-player games in [Bilo and Mavronicolas, Proceedings of SAGT 2012; Conitzer and Sandholm, Games and Economic Behavior, 2008; Gilboa and Zemel, Games and Economic Behavior, 1989]. Encompassed in the catalog are the four decision problems shown EXISTS-R-complete in [Garg et al., Proceedings of ICALP 2015].
@InProceedings{bilo_et_al:LIPIcs.STACS.2016.17,
author = {Bil\`{o}, Vittorio and Mavronicolas, Marios},
title = {{A Catalog of EXISTS-R-Complete Decision Problems About Nash Equilibria in Multi-Player Games}},
booktitle = {33rd Symposium on Theoretical Aspects of Computer Science (STACS 2016)},
pages = {17:1--17:13},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-001-9},
ISSN = {1868-8969},
year = {2016},
volume = {47},
editor = {Ollinger, Nicolas and Vollmer, Heribert},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.STACS.2016.17},
URN = {urn:nbn:de:0030-drops-57189},
doi = {10.4230/LIPIcs.STACS.2016.17},
annote = {Keywords: Nash equilibrium, complexity of equilibria, EXISTS-R-completeness}
}