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Every uncountable Solovay-model set of reals has a perfect subset
Statement
In , every uncountable subset of contains a nonempty perfect subset.
Facts & Assumptions
Given: Uncountable 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 inaccessible Lévy-collapse setup for Solovay's construction, and Valuation of names and M[G]: reals localize to bounded collapse stages; the initial stages factor by disjoint coordinates; and an element of a generic extension is the valuation of a ground-model name.
The Solovay inner model is closed under ambient omega-sequences: an ambient enumeration with values in belongs to .
Forcing theorem, Monotonicity, density, and decision for forcing, and Absorption, factorization, and homogeneous truth in the Solovay collapse: the forcing relation is definable, a supplied generic meets each ground dense set, and a true localized membership assertion is forced below a condition and is fixed by the homogeneous tail.
A perfect tree of mutually generic name interpretations: a condition forcing a new real in yields a perfect image.
Borel-code, measure, category, and perfect-set absoluteness: the coded perfect image transfers to .
Proof
Use F2 to choose such that contains F1's real parameter; keep the finite ordinal parameters explicit. Fix an ambient enumeration . If , choose one (available because is uncountable) and define when , and otherwise. This is an ambient omega-surjection onto with values in , so F3 puts it in , contradicting internal uncountability. Hence some exists.
Use F2 again to choose with and . Let be the finite-condition collapse on and . The disjoint-coordinate projection in F2 gives , with and small. By the defining valuation formula for in F2, choose a -name with .
In form the downward-open set . The actual generic misses , since the truth lemma would otherwise put in . The set is dense and belongs to , so genericity gives incompatible with every member of . Hence no extension of forces equal to a real of . Separately, the truth lemma and tail homogeneity give forcing the fixed real-membership formula defining (with its localized real and ordinal parameters). Directedness of gives below . This has the exact forcing-newness premise of F5 and forces every branch interpretation to satisfy the definition of ; no external class formula “” has been used in the forcing language.
Apply F5 below . Every branch interpretation satisfies the same definition, so its compact injective image lies in . The construction has a real code; since has all reals, that code lies in , and F6 says internally that is nonempty perfect.
Depends on
- The Solovay inner model satisfies ZF and every real set has a real–ordinal definition
- The Lévy collapse localizes countable ordinal data
- The inaccessible Lévy-collapse setup for Solovay's construction
- Valuation of names and M[G]
- Absorption, factorization, and homogeneous truth in the Solovay collapse
- The Solovay inner model is closed under ambient omega-sequences
- Forcing theorem
- Monotonicity, density, and decision for forcing
- Borel-code, measure, category, and perfect-set absoluteness
- A perfect tree of mutually generic name interpretations
Used by
Dependency tree · two levels
38 results within two dependency steps of this one, each drawn at its shortest distance from it. An arrow runs from a result to what uses it, so the chart reads left to right and ends at this result, which carries a heavier outline. Every node is a link to that result. Click elsewhere on the chart to enlarge it.
Sources
- Solovay 1970, Part III, Lemmas 1.6 and 2.11 (standard reference, not scraped)