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Complexity of parity game solving compared to PLS, PPA, and PPAD

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Since parity game solving is in TFNP ("Total Function Nondeterministic Polynomial") (and the decision version is is NP ∩ coNP), I wonder whether it is contained in PLS ("Polynomial Local Search") or PPA ("Polynomial Parity Argument")? Add PPP ("Polynomial Pigeonhole Principle") if you want, even so this would probably mean that it is already contained in PPAD ("Polynomial Parity Arguments on Directed graphs"), and hence in PPA.

Or is it rather the other way round and parity game solving can be shown to be hard for PLS or PPAD? But that would be surprising, since a recursive algorithm that solves parity games is known (even if it is not efficient in the worst case).

Asked By : Thomas Klimpel
Answered By : Rahul Savani

Yes, solving parity games is known to be in PPAD (and thus PPA and PPP too) and PLS, and is thus unlikely to be hard for either (since this would imply containment of one of these classes in the other).

See, e.g.,

Daskalakis, Constantinos, and Christos Papadimitriou. "Continuous local search." Proceedings of the twenty-second annual ACM-SIAM symposium on Discrete Algorithms. SIAM, 2011.

and combine the membership of Simple Stochastic Games (SSGs) in CLS (which is in PPAD and PLS) with the well-known observation that solving parity games can be reduced to solving SSGs in polynomial time.

The reason that these problems are in PPAD is that they admit "optimality equations", rather like Bellman equations, that characterize solutions as fixed points. The reason these problems are in PLS is that they can be solved with local improvement algorithms like strategy improvement (a two-player generalization of policy iteration for MDPs).

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