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LemmaStatement: AI-adaptedProof: AI-adaptedprecheck passverified 2026-08-09 (gpt-5.6-terra-codex-subscription)
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The first chip identifies the source of the statement or construction; the second identifies the source of its local proof or verification.

  • Literature-sourced: the exact statement appears in a cited source; only wording and notation differ.
  • AI-adapted: a semantically identical restatement of literature-sourced material, modulo indexing, notation, and boundary cases adopted by the library.
  • AI-generated: a genuinely novel statement formulated by AI, with no source for the claim itself.

These labels describe origin, not correctness: citations and verification chips remain separate evidence.

Under choice, every open cover of a paracompact Hausdorff space has locally finite open refinements {Vs} and {Ws} with Vs‾⊆Ws⊆Ws‾⊆Us

Statement

Assume the Axiom of Choice. If X is paracompact and Hausdorff and U is an open cover, there are a set S, a map s↦Us from S into U, and locally finite open covers {Vs}s∈S and {Ws}s∈S with Vs‾⊆Ws⊆Ws‾⊆Us(s∈S).

Facts & Assumptions

Given: The Axiom of Choice, a paracompact Hausdorff space X, and an open cover U.

[A1]

Every family of nonempty sets has a choice function (The Axiom of Choice).

[L3]

Proof

technique · constructive
1.1A1L1L3F1construct

We first prove a one-shrink construction for any open cover C. Let R be the family of all open R for which R‾⊆C for some C∈C. By [L1] and [L3], R covers X. Take a locally finite open refining cover A of R by [F1], discard its empty members, and use [A1] to assign to each A∈A sets R(A)∈R and C(A)∈C with A⊆R(A)⊆R(A)‾⊆C(A). Then A‾⊆R(A)‾⊆C(A).

2.1step 1.1

Apply step 1.1 to U. This gives a locally finite open cover {Ws}s∈S and assigned Us∈U such that Ws‾⊆Us.

3.1step 1.1step 2.1construct

Apply step 1.1 again, now to the cover {Ws:s∈S}. Obtain a locally finite open cover {At}t∈T and a map t↦s(t) such that At‾⊆Ws(t). For s∈S put Vs:=⋃{At:s(t)=s}. The family {Vs}s∈S is an open cover. It is locally finite because any neighbourhood meeting only finitely many At meets only the corresponding finitely many grouped unions Vs.

4.1L2step 2.1step 3.1discharge-construct∎

Each subfamily {At:s(t)=s} is locally finite, so [L2] gives Vs‾=⋃s(t)=sAt‾⊆Ws. Together with step 2.1 this yields Vs‾⊆Ws⊆Ws‾⊆Us for every s, with both displayed families locally finite open covers.

Remarks

The Axiom of Choice is used to retain the assignments to cover members through the two locally finite refinements. This is a sufficient hypothesis for this construction; no claim is made that it is the exact choice strength.

Depends on

Dependency tree · 2 levels

Sources