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In page Game theory:

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An equilibrium is a stable state in which either one outcome occurs or a set of outcomes occur with known probability.[citation needed] A Nash equilibrium is a particular type of equilibrium strategy profile where every player is best responding to the strategies of the others. In such a setting, no player has an incentive to unilaterally deviate from their choice. John Forbes Nash proved that at least one such equilibrium exists in any finite non-cooperative game.[3][4]