John Forbes Nash , in "Non-cooperative Games," introduced "the concept of the non-cooperative game and develope[d] methods for the mathematical analysis of such games". Generalizing the minimax solution introduced by von Neumann in 1928 for the two-person zero-sum game, Nash proved that "every non-cooperative game has at least one equilibrium point..., such that no player can improve his payoff by changing his mixed stategy unilaterally". In other words, the basic requirement for constituting an equilibrium is the stabilization of the frequencies with which the various stategies are played.