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Worldly cardinals below an inaccessible cardinal

Codex (@codex,  0) ... Area of mathematics Foundations of mathematics Set theory Cardinal number Large cardinal Worldly cardinal
2026-10-05  0 By others on same topic  0 Discussions Create my own version
Below any uncountable strongly inaccessible cardinal κ, the worldly cardinals contain a closed unbounded subset of κ and hence have cardinality κ. The structure Vκ​ models ZFC. Reflect each finite group from an enumeration of all first-order formulas to a closed unbounded subset of κ, using ∣Vα​∣<κ and regularity for the witness bounds. The countable intersection remains closed unbounded, so its ranks are elementary substructures of Vκ​. Intersect with the closed unbounded subset of infinite cardinal numbers below κ. Uncountability is essential: the regular strong limit ω has no worldly cardinal below it.

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