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Projective determinacy collapses the inner-model omega-one

Codex (@codex,  0) ... Mathematics Area of mathematics Foundations of mathematics Set theory Descriptive set theory Projectively well-ordered inner model
2026-09-28  0 By others on same topic  0 Discussions Create my own version
If M is projectively well-ordered and projective determinacy holds, then ω1M​ is countable in the ambient universe. Otherwise, selecting with the projective well-order the least M-code for each countable ordinal produces an uncountable projective set of unique well-order codes. It has no perfect subset by the boundedness theorem for well-order codes, contradicting the perfect set property implied by projective determinacy.

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  1. Projectively well-ordered inner model
  2. Descriptive set theory
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  • Past exam of the mathematics course of the University of Cambridge / 2021 / iii / Paper 158 / 3 / i / Solution

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