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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 AN there are in N an open set U and a meagre set M in the relevant space such that AUM.

Facts & Assumptions

Given: A set AR with AN in the ambient Shelah extension.

[F1]

The Shelah HOD(S) model and its real-ordinal presentation: A has a rank-bounded definition from one sS 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 D2ω of sequences with infinitely many 1s, with countable complement. Baire sequence space is homeomorphic to the irrational real numbers identifies ωω with RQ, 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

1.1

First let A2ω belong to N. By [F1], A has a rank-bounded definition from one parameter sS 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 ACcE0. This uses the claim as written; no open set is read directly from the Borel representative.

F1F2
1.2

This proves the all-Baire-property assertion for the standard set-theoretic real space 2ω. To compare with the usual real line, let D2ω 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:DRQ.

F3F6
2.1

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 CcUu. Pair the Borel/open code and both meagre-error sequences into finitely many binary reals. By [F3], every such code real belongs to N.

F3F5step 1.1
3.1

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.

F3F7step 2.1
4.1

Membership in AN 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 NAUuE0Ec. 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.

F3F4step 1.1step 2.1step 3.1
5.1

Let AR belong to N. The coded set A=h1[A(RQ)]D, viewed as a subset of 2ω by putting no points outside D, belongs to N. Step 4.1 gives AU meagre in 2ω. Restrict to the dense subspace D and transport by h: the image differs from the relatively open set h[UD] by a meagre subset of RQ. 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(RQ) for an open WR. Adding the countable rational set shows AW is meagre in R. All maps, countable complements and codes used here are the fixed objects of [F6] and belong to N.

F3F4F6step 4.1step 1.2
6.1

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.

F3step 4.1step 5.1

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