Determinacy | Theorems in the foundations of mathematics

Borel determinacy theorem

In descriptive set theory, the Borel determinacy theorem states that any Gale–Stewart game whose payoff set is a Borel set is determined, meaning that one of the two players will have a winning strategy for the game. A Gale-Stewart game is a possibly infinite two-player game, where both players have perfect information and no randomness is involved. The theorem is a far reaching generalization of Zermelo's Theorem about the determinacy of finite games. It was proved by Donald A. Martin in 1975, and is applied in descriptive set theory to show that Borel sets in Polish spaces have regularity properties such as the perfect set property and the property of Baire. The theorem is also known for its metamathematical properties. In 1971, before the theorem was proved, Harvey Friedman showed that any proof of the theorem in Zermelo–Fraenkel set theory must make repeated use of the axiom of replacement. Later results showed that stronger determinacy theorems cannot be proven in Zermelo–Fraenkel set theory, although they are relatively consistent with it, if certain large cardinals are consistent. (Wikipedia).

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Related pages

Cantor space | Axiom of projective determinacy | Set theory | Measurable cardinal | Topology | Borel hierarchy | Cumulative hierarchy | Baire space | Game tree | Zermelo–Fraenkel set theory | Descriptive set theory | Borel set | Determinacy | Large cardinal | Perfect set property | Tree (descriptive set theory) | Zermelo's theorem (game theory) | Closed set | Axiom of determinacy | Subset | Property of Baire | Analytic set | Polish space | Power set | Metamathematics | Product topology | Open set