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TheoremStatement: Literature-sourcedProof: AI-adaptedPipeline-generatedaudited 2026-09-22
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Every real set in the Shelah inner model has the Baire property

Statement

N satisfies: every subset of the reals has the property of Baire. Here the set-theoretic reals are represented first by Cantor space 2ω; the same assertion for the usual real line follows through comeagre homeomorphic coding subspaces. Explicitly, for each real set A∈N there are in N an open set U and a meagre set M in the relevant space such that A△U⊆M.

Facts & Assumptions

Given: A set A⊆R with A∈N in the ambient Shelah extension.

[F1]

The Shelah HOD(S) model and its real-ordinal presentation: A has a rank-bounded definition from one s∈S and finitely many ordinals; over the constructible ground this is equivalent to definability from one real and finitely many ordinals.

[F2]

Strongly homogeneous truth has Baire representatives: for every formula with a countable ordinal-sequence parameter, the set of binary reals satisfying it differs from a Borel set by a coded meagre set; its proof places the Boolean truth value in the countably generated parameter-and-Cohen algebra by free amalgamation and transports its Borel reading by automorphism extension.

[F3]

The Shelah inner model satisfies ZF and Dependent Choice: N is transitive and has the same reals as the ambient extension.

[F4]

The property of Baire: the property of Baire is the existence of an open set differing from the given set by a meagre set.

[F5]

Borel-code, measure, category, and perfect-set absoluteness: from a Borel code one uniformly obtains an open code and a coded sequence of closed nowhere-dense sets covering their symmetric difference. Its stated evaluation-absoluteness interface is restricted to Solovay intermediate models, so the proof below does not apply that clause to N.

[F6]

Cantor and Baire sequence spaces and coordinate codings gives a homeomorphism from ωω onto the subspace D⊆2ω of sequences with infinitely many 1s, with countable complement. Baire sequence space is homeomorphic to the irrational real numbers identifies ωω with R∖Q, and Q is countably infinite makes the omitted rational set countable.

[F7]

Well-founded Borel evaluation codes: a Borel code is a real-coded countable labelled tree whose child relation is well-founded, and evaluation proceeds through leaf, complement and countable-union nodes.

Proof

1.1F1F2

First let A⊆2ω belong to N. By [F1], A has a rank-bounded definition from one parameter s∈S and finitely many ordinals. Applying [F2] to that exact defining formula gives a Borel code c and a coded meagre set E0 in the ambient extension with A△Cc⊆E0. This uses the claim as written; no open set is read directly from the Borel representative.

1.2F3F6

This proves the all-Baire-property assertion for the standard set-theoretic real space 2ω. To compare with the usual real line, let D⊆2ω be the infinitely-many-1s subspace of [F6]. Its complement is explicitly at most countable and hence meagre; likewise Q is countable and meagre in R. Composing the two homeomorphisms in [F6] gives h:D⟶R∖Q.

2.1F3F5step 1.1

Apply only the uniform construction clause of [F5] to c. It yields an open code u and a coded sequence of closed nowhere-dense sets covering Cc△Uu. Pair the Borel/open code and both meagre-error sequences into finitely many binary reals. By [F3], every such code real belongs to N.

3.1F3F7step 2.1

We verify the needed absoluteness directly, rather than use the Solovay-intermediate-model clause of [F5]. A code from [F7] is a labelled tree on ω<ω and hence a real. If its child relation were ill-founded in either of the two transitive same-real models, DC in N (and Choice in the ambient extension) would produce a descending sequence of nodes, itself a real; therefore well-foundedness agrees. For a shared real x, if the two evaluations first differed at a node, an R-minimal such node would have agreeing child evaluations, and the leaf, complement and union rules would force agreement at that node, a contradiction. Thus Cc and Uu have identical evaluations on the common reals. For a binary tree code, closedness is immediate and nowhere density is the arithmetic finite-cylinder test: every finite word has an extension above which some finite level has no tree node. That test is absolute, so every displayed closed-nowhere-dense code remains such in N.

4.1F3F4step 1.1step 2.1step 3.1

Membership in A∈N is absolute between the transitive model N and the ambient extension. Hence the ambient inclusions from steps 1.1 and 2.1, together with step 3.1, give N⊨A△Uu⊆E0∪Ec. Dependent Choice in [F3] supplies Countable Choice, and the two actual coded sequences of nowhere-dense sets therefore witness in N that the right side is meagre.

5.1F3F4F6step 4.1step 1.2

Let A⊆R belong to N. The coded set A∗=h−1[A∩(R∖Q)]⊆D, viewed as a subset of 2ω by putting no points outside D, belongs to N. Step 4.1 gives A∗△U meagre in 2ω. Restrict to the dense subspace D and transport by h: the image differs from the relatively open set h[U∩D] by a meagre subset of R∖Q. A nowhere-dense subset of a dense subspace is nowhere dense in the whole space after taking ambient closure, so that error is meagre in R. Write the relatively open image as W∩(R∖Q) for an open W⊆R. Adding the countable rational set shows A△W is meagre in R. All maps, countable complements and codes used here are the fixed objects of [F6] and belong to N.

6.1F3step 4.1step 5.1∎

Since A was arbitrary, steps 4.1 and 5.1 prove the assertion for both the set-theoretic and usual-real conventions, with witnesses in N. This is the Statement.

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