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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.

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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
1.1

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).

A1L1L3F1construct
2.1

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

step 1.1
3.1

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.

step 1.1step 2.1construct
4.1

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.

L2step 2.1step 3.1discharge-construct∎

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

Used by

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Sources