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Every real set in the Shelah inner model has the Baire property
Statement
satisfies: every subset of the reals has the property of Baire. Here the set-theoretic reals are represented first by Cantor space ; the same assertion for the usual real line follows through comeagre homeomorphic coding subspaces. Explicitly, for each real set there are in an open set and a meagre set in the relevant space such that .
Facts & Assumptions
Given: A set with in the ambient Shelah extension.
The Shelah HOD(S) model and its real-ordinal presentation: has a rank-bounded definition from one and finitely many ordinals; over the constructible ground this is equivalent to definability from one real and finitely many ordinals.
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.
The Shelah inner model satisfies ZF and Dependent Choice: is transitive and has the same reals as the ambient extension.
The property of Baire: the property of Baire is the existence of an open set differing from the given set by a meagre set.
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 .
Cantor and Baire sequence spaces and coordinate codings gives a homeomorphism from onto the subspace of sequences with infinitely many s, with countable complement. Baire sequence space is homeomorphic to the irrational real numbers identifies with , and is countably infinite makes the omitted rational set countable.
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
First let belong to . By [F1], has a rank-bounded definition from one parameter and finitely many ordinals. Applying [F2] to that exact defining formula gives a Borel code and a coded meagre set in the ambient extension with . This uses the claim as written; no open set is read directly from the Borel representative.
This proves the all-Baire-property assertion for the standard set-theoretic real space . To compare with the usual real line, let be the infinitely-many-s subspace of [F6]. Its complement is explicitly at most countable and hence meagre; likewise is countable and meagre in . Composing the two homeomorphisms in [F6] gives
Apply only the uniform construction clause of [F5] to . It yields an open code and a coded sequence of closed nowhere-dense sets covering . Pair the Borel/open code and both meagre-error sequences into finitely many binary reals. By [F3], every such code real belongs to .
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 and hence a real. If its child relation were ill-founded in either of the two transitive same-real models, DC in (and Choice in the ambient extension) would produce a descending sequence of nodes, itself a real; therefore well-foundedness agrees. For a shared real , if the two evaluations first differed at a node, an -minimal such node would have agreeing child evaluations, and the leaf, complement and union rules would force agreement at that node, a contradiction. Thus and 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 .
Membership in is absolute between the transitive model and the ambient extension. Hence the ambient inclusions from steps 1.1 and 2.1, together with step 3.1, give Dependent Choice in [F3] supplies Countable Choice, and the two actual coded sequences of nowhere-dense sets therefore witness in that the right side is meagre.
Let belong to . The coded set , viewed as a subset of by putting no points outside , belongs to . Step 4.1 gives meagre in . Restrict to the dense subspace and transport by : the image differs from the relatively open set by a meagre subset of . 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 . Write the relatively open image as for an open . Adding the countable rational set shows is meagre in . All maps, countable complements and codes used here are the fixed objects of [F6] and belong to .
Since was arbitrary, steps 4.1 and 5.1 prove the assertion for both the set-theoretic and usual-real conventions, with witnesses in . This is the Statement.
Depends on
- The Shelah inner model satisfies ZF and Dependent Choice
- Strongly homogeneous truth has Baire representatives
- Borel-code, measure, category, and perfect-set absoluteness
- Well-founded Borel evaluation codes
- Cantor and Baire sequence spaces and coordinate codings
- Baire sequence space is homeomorphic to the irrational real numbers
- $\mathbb{Q}$ is countably infinite
- The property of Baire
- The Shelah HOD(S) model and its real-ordinal presentation
Used by
Dependency tree · two levels
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Sources
- Robert M. Solovay, A Model of Set-Theory in Which Every Set of Reals Is Lebesgue Measurable (standard reference, not scraped)
- Saharon Shelah, Can You Take Solovay's Inaccessible Away? (standard reference, not scraped)