Abstract
We study the problem of finding approximate Nash equilibria that satisfy certain conditions, such as providing good social welfare. In particular, we study the problem \(\epsilon \)-NE \(\delta \)-SW: find an \(\epsilon \)-approximate Nash equilibrium (\(\epsilon \)-NE) that is within \(\delta \) of the best social welfare achievable by an \(\epsilon \)-NE. Our main result is that, if the randomized exponential-time hypothesis (RETH) is true, then solving \(\left( \frac{1}{8} - \mathrm {O}(\delta )\right) \)-NE \(\mathrm {O}(\delta )\)-SW for an \(n\times n\) bimatrix game requires \(n^{\mathrm {\widetilde{\Omega }}(\delta ^{\varLambda } \log n)}\) time, where \(\varLambda \) is a constant.
Building on this result, we show similar conditional running time lower bounds on a number of decision problems for approximate Nash equilibria that do not involve social welfare, including maximizing or minimizing a certain player’s payoff, or finding approximate equilibria contained in a given pair of supports. We show quasi-polynomial lower bounds for these problems assuming that RETH holds, and these lower bounds apply to \(\epsilon \)-Nash equilibria for all \(\epsilon < \frac{1}{8}\). The hardness of these other decision problems has so far only been studied in the context of exact equilibria.
The authors were supported by EPSRC grant EP/L011018/1. The full version of this paper, with complete proofs, is available at http://arxiv.org/abs/1608.03574.
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Notes
- 1.
While the proof in [6] produces a lower bound for 0.8-NE \((1 - \mathrm {O}(\epsilon ))\)-SW, this is in a game with maximum payoff \(\mathrm {O}(1/\epsilon )\). Therefore, when the payoffs in this game are rescaled to [0, 1], the resulting lower bound only applies to \(\epsilon \)-NE \(\epsilon \)-SW.
- 2.
- 3.
Here \(\mathrm {\widetilde{\Omega }}(\log n)\) means \(\mathrm {\Omega }(\frac{\log n}{(\log \log n)^c})\) for some constant c.
- 4.
If |Y| is not even, then we can create a new free game in which each question in |Y| appears twice. This will not change the value of the free game.
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Deligkas, A., Fearnley, J., Savani, R. (2016). Inapproximability Results for Approximate Nash Equilibria. In: Cai, Y., Vetta, A. (eds) Web and Internet Economics. WINE 2016. Lecture Notes in Computer Science(), vol 10123. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-662-54110-4_3
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