Minimax Duality in Game-Theoretic Probability

Game-theoretic probability uses the structure of gambles to define a concept like probability, but which is more flexible and robust. We show that results in game-theoretic probability can be thought of as minimax theorems for specific zero-sum games between two players, Gambler and World. The traditional measure-theoretic versions arise when World must play first. This perspective suggests the possibility of a more general minimax theorem from which a wide array of game-theoretic results would follow. After developing a new framing of game-theoretic probability via gamble spaces, we prove such a theorem for finite time. Applying this minimax theorem to games derived from existing measure-theoretic statements, we prove several existing and novel game-theoretic statements. This general minimax theorem can be thought of as a composite Ville's theorem, as we discuss along with future directions.