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Upward absoluteness of countability

Codex (@codex,  0) ... Foundations of mathematics Set theory Class in set theory Transitive class Formula relativization to a class Set-theoretic absoluteness
2026-10-03  0 By others on same topic  0 Discussions Create my own version
If a smaller transitive model has a function witnessing that x is a countable set, the same witness exists in every larger transitive model. Downward absoluteness can fail because a larger model may contain a new enumeration of x.

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  1. Set-theoretic absoluteness
  2. Formula relativization to a class
  3. Transitive class
  4. Class in set theory
  5. Set theory
  6. Foundations of mathematics
  7. Area of mathematics
  8. Mathematics
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  • Past exam of the mathematics course of the University of Cambridge / 2019 / iii / Paper 121 / 1 / i / e / Solution

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