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TheoremStatement: Literature-sourcedProof: AI-adaptedPipeline-generatedaudited 2026-09-14
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Every uncountable Solovay-model set of reals has a perfect subset

Statement

In M, every uncountable subset of R contains a nonempty perfect subset.

Facts & Assumptions

Given: Uncountable AR in M.

[F1]

The Solovay inner model satisfies ZF and every real set has a real–ordinal definition: gives a definition of A from one real and finitely many ordinals.

[F2]

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.

[F3]

The Solovay inner model is closed under ambient omega-sequences: an ambient enumeration with values in M belongs to M.

[F4]

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.

[F5]

A perfect tree of mutually generic name interpretations: a condition forcing a new real in A yields a perfect image.

[F6]

Borel-code, measure, category, and perfect-set absoluteness: the coded perfect image transfers to M.

Proof

1.1

Use F2 to choose ξ<κ such that N=V[Gξ] contains F1's real parameter; keep the finite ordinal parameters explicit. Fix an ambient enumeration e:ωRN. If AN, choose one a0A (available because A is uncountable) and define eA(n)=e(n) when e(n)A, and eA(n)=a0 otherwise. This is an ambient omega-surjection onto A with values in M, so F3 puts it in M, contradicting internal uncountability. Hence some xAN exists.

F1F2F3
1.2

Use F2 again to choose η with ξ<η<κ and xV[Gη]. Let Q be the finite-condition collapse on [ξ,η)×ω and H=Gη([ξ,η)×ω). The disjoint-coordinate projection in F2 gives V[Gη]=N[H], with QN and Q small. By the defining valuation formula for N[H] in F2, choose a Q-name τ˙N with τ˙H=x.

2.1

In N form the downward-open set D={qQ:(yRN) qQτ˙=yˇ}. The actual generic H misses D, since the truth lemma would otherwise put x=τ˙H in N. The set D{q:qD} is dense and belongs to N, so genericity gives p0H incompatible with every member of D. Hence no extension of p0 forces τ˙ equal to a real of N. Separately, the truth lemma and tail homogeneity give p1H forcing the fixed real-membership formula defining A (with its localized real and ordinal parameters). Directedness of H gives pH below p0,p1. This p has the exact forcing-newness premise of F5 and forces every branch interpretation to satisfy the definition of A; no external class formula “τ˙N” has been used in the forcing language.

F1F2F4step 1.1step 1.2
3.1

Apply F5 below p. Every branch interpretation satisfies the same definition, so its compact injective image P lies in A. The construction has a real code; since M has all reals, that code lies in M, and F6 says internally that P is nonempty perfect.

F5F6step 2.1

Depends on

Used by

Dependency tree · two levels

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Sources