This definition applies to games of two or more players, and Nash showed that the various definitions of "solutions" for games that had been given earlier all yield Nash equilibria.
As a simple example, consider the following twoplayer game: both players simultaneously choose a whole number between 0 and 10, inclusive. Both players then win the minimum of the two numbers in dollars. In addition, if one player chose a larger number than the other, then he has to pay $2 to the other. This game has a unique Nash equilibrium: both players have to choose 0. Any other choice of strategies can be improved if one of the players lowers his number. If the game is modified so that the two players win the named amount if they both choose the same number, and otherwise win nothing, then there are 11 Nash equilibria.
If a game has a unique Nash equilibrium and is played among completely rational players (who know that all players are completely rational), then the players will choose the strategies that form the equilibrium.
A game may have many Nash equilibria, or none. Nash was able to prove that, if we allow mixed strategies (players choose strategies randomly according to preassigned probabilities), then every nplayer game in which every player can chose from finitely many strategies admits at least one Nash equilibrium of mixed strategies.
See also: Prisoner's dilemma, Evolutionarily stable strategy.
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