Alphabeta Math
LemmaStatement: AI-adaptedProof: AI-adaptedSession-authored (Fable 5 assisted)precheck passverified 2026-08-09 (gpt-5.6-terra-codex-subscription)
How statement and proof provenance work

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}\{V_s\} and {Ws}\{W_s\} with VsWsWsUs\overline{V_s}\subseteq W_s\subseteq\overline{W_s}\subseteq U_s

Statement

Assume the Axiom of Choice. If XX is paracompact and Hausdorff and U\mathcal U is an open cover, there are a set SS, a map sUss\mapsto U_s from SS into U\mathcal U, and locally finite open covers {Vs}sS\{V_s\}_{s\in S} and {Ws}sS\{W_s\}_{s\in S} with VsWsWsUs(sS).\overline{V_s}\subseteq W_s\subseteq\overline{W_s}\subseteq U_s\quad(s\in S).

Facts & Assumptions

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

[A1]

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

[L3]

In a regular space, xOx\in O open gives an open RR with xRROx\in R\subseteq\overline R\subseteq O (A space is regular if and only if every point has a neighbourhood base of closed neighbourhoods, if and only if xUx \in U open gives an open VV with xVVUx \in V \subseteq \overline{V} \subseteq U, implication (a)\Rightarrow(b)).

Proof

technique · constructive
1.1

We first prove a one-shrink construction for any open cover C\mathcal C. Let R\mathcal R be the family of all open RR for which RC\overline R\subseteq C for some CCC\in\mathcal C. By [L1] and [L3], R\mathcal R covers XX. Take a locally finite open refining cover A\mathcal A of R\mathcal R by [F1], discard its empty members, and use [A1] to assign to each AAA\in\mathcal A sets R(A)RR(A)\in\mathcal R and C(A)CC(A)\in\mathcal C with AR(A)R(A)C(A).A\subseteq R(A)\subseteq\overline{R(A)}\subseteq C(A). Then AR(A)C(A)\overline A\subseteq\overline{R(A)}\subseteq C(A).

A1L1L3F1construct
2.1

Apply step 1.1 to U\mathcal U. This gives a locally finite open cover {Ws}sS\{W_s\}_{s\in S} and assigned UsUU_s\in\mathcal U such that WsUs\overline{W_s}\subseteq U_s.

step 1.1
3.1

Apply step 1.1 again, now to the cover {Ws:sS}\{W_s:s\in S\}. Obtain a locally finite open cover {At}tT\{A_t\}_{t\in T} and a map ts(t)t\mapsto s(t) such that AtWs(t)\overline{A_t}\subseteq W_{s(t)}. For sSs\in S put Vs:={At:s(t)=s}.V_s:=\bigcup\{A_t:s(t)=s\}. The family {Vs}sS\{V_s\}_{s\in S} is an open cover. It is locally finite because any neighbourhood meeting only finitely many AtA_t meets only the corresponding finitely many grouped unions VsV_s.

step 1.1step 2.1construct
4.1

Each subfamily {At:s(t)=s}\{A_t:s(t)=s\} is locally finite, so [L2] gives Vs=s(t)=sAtWs.\overline{V_s} =\bigcup_{s(t)=s}\overline{A_t}\subseteq W_s. Together with step 2.1 this yields VsWsWsUs\overline{V_s}\subseteq W_s\subseteq\overline{W_s}\subseteq U_s for every ss, 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

Dependency tree · next 3 levels

Direct dependencies and their dependencies through the next three levels: 28 results over 9 levels. An arrow runs from a result to what uses it, and this result sits at the bottom with a heavier outline. Click the chart to enlarge it.

Sources