Abstract
In 1964 Shapley observed that a matrix has a saddle point in pure strategies whenever every its \(2 \times 2\) submatrix has one. In contrast, a bimatrix game may have no pure strategy Nash equilibrium (NE) even when every \(2 \times 2\) subgame has one. Nevertheless, Shapley’s claim can be extended to bimatrix games as follows. We partition all \(2 \times 2\) bimatrix games into fifteen classes \(C = \{c_1, \ldots , c_{15}\}\) depending only on the preferences of two players. A subset \(t \subseteq C\) is called a NE-theorem if a bimatrix game has a NE whenever it contains no subgame from t. We suggest a method to construct all minimal (that is, strongest) NE-theorems based on the procedure of joint generation of transversal hypergraphs given by a special oracle. By this method we obtain all (six) strongest NE-theorems. Let us remark that the suggested approach, which may be called “math-pattern recognition”, is very general: it allows to characterize completely an arbitrary “target” in terms of arbitrary “attributes”.
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Acknowledgments
We are thankful to Bernhard von Stengel and two anonymous referees for careful reading and many helpful remarks and suggestions. We thank also Nikolai Kukushkin who promoted the idea of generalizing Shapley’s (1964) theorem to bimatrix games and various concepts of solution. The first author gratefully acknowledges partial support by the National Science Foundation (Grant IIS-1161476).
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Boros, E., Elbassioni, K., Gurvich, V. et al. Sufficient conditions for the existence of Nash equilibria in bimatrix games in terms of forbidden \(2 \times 2\) subgames. Int J Game Theory 45, 1111–1131 (2016). https://doi.org/10.1007/s00182-015-0513-7
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DOI: https://doi.org/10.1007/s00182-015-0513-7
Keywords
- Matrix and bimatrix games
- Saddle point
- Nash equilibrium
- Nash-solvability
- Dual hypergraphs
- Transversal hypergraphs
- Dualization