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We study multi-strategies in multiplayer reachability games played on finite graphs. A multi-strategy prescribes a set of possible actions, instead of a single action as usual strategies: it represents a set of all strategies that are consistent with it. We aim for profiles of multi-strategies (a multi-strategy per player), where each profile of consistent strategies is a Nash equilibrium, or a subgame perfect equilibrium. The permissiveness of two multi-strategies can be compared with penalties, as already used in the two-player zero-sum setting by Bouyer, Duflot, Markey and Renault [Patricia Bouyer et al., 2009]. We show that we can decide the existence of a multi-strategy profile that is a Nash equilibrium or a subgame perfect equilibrium, while satisfying some upper-bound constraints on the penalties in PSPACE, if the upper-bound penalties are given in unary. The same holds when we search for multi-strategies where certain players are asked to win in at least one play or in all plays.
@InProceedings{goeminne_et_al:LIPIcs.CSL.2025.23,
author = {Goeminne, Aline and Monmege, Benjamin},
title = {{Permissive Equilibria in Multiplayer Reachability Games}},
booktitle = {33rd EACSL Annual Conference on Computer Science Logic (CSL 2025)},
pages = {23:1--23:17},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-362-1},
ISSN = {1868-8969},
year = {2025},
volume = {326},
editor = {Endrullis, J\"{o}rg and Schmitz, Sylvain},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CSL.2025.23},
URN = {urn:nbn:de:0030-drops-227801},
doi = {10.4230/LIPIcs.CSL.2025.23},
annote = {Keywords: multiplayer reachability games, penalties, permissive equilibria}
}