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Every set of reals in the Solovay model has the Baire property
Statement
In , every subset of has the property of Baire.
Facts & Assumptions
Given: in .
The Solovay inner model satisfies ZF and every real set has a real–ordinal definition: gives a definition of from one real and finitely many ordinals.
The Lévy collapse localizes countable ordinal data: the real parameter lies in a bounded intermediate model whose relevant codes are countable in the final extension.
Homogeneous truth about a generic real has Borel representatives gives an -coded Borel agreeing with on every -Cohen generic, while Random and Cohen generics over an intermediate model are conull and comeagre says that the nongeneric reals form an ambient meagre set.
Borel-code, measure, category, and perfect-set absoluteness and The property of Baire: coded Borel sets are open modulo coded meagre sets, absolutely, and internal DC closes the meagre ideal countably.
Proof
Use F2 to choose a bounded containing F1's real parameter; the ordinal parameters remain explicit. F3 gives an -coded Borel set agreeing with on every -Cohen generic. In the ambient final extension, enumerate the -coded closed nowhere-dense sets as , as in the proof of F3's generic-largeness component, and let be the real Borel code for . Every nongeneric real lies in , so . The code need not lie in , but F1 says that and the final extension have the same reals, hence ; F4 makes its evaluation and meagreness absolute to . Apply F4 inside to the code of to obtain a coded open and coded meagre with .
The explicit codes for and lie in , and F4 closes the meagre ideal under their finite union (equivalently, interleave their two coded nowhere-dense witness sequences). Thus is meagre in , which is exactly BP. Empty and whole-space cases use and .
Depends on
- The Solovay inner model satisfies ZF and every real set has a real–ordinal definition
- The Lévy collapse localizes countable ordinal data
- Borel-code, measure, category, and perfect-set absoluteness
- Random and Cohen generics over an intermediate model are conull and comeagre
- Homogeneous truth about a generic real has Borel representatives
- The property of Baire
Used by
Dependency tree · two levels
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Sources
- Solovay 1970, Part III, Lemmas 1.5 and 2.10 (standard reference, not scraped)