Zermelo's theorem
Zermelo’s theorem states that in a finite two-player game of perfect information with no chance, such as Chess, one player can force a win or both players can force a draw. It does not by itself provide the strategy.
Chess, Go, and gomoku are proven to have unbeatable strategies (Zermelo's theorem). The theorem proves they exist—not what they are. In gomoku, the winning strategy is known, so tournaments use modified rules.
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