Alphabeta Math
LemmaStatement: Literature-sourcedProof: AI-adaptedprecheck passjudge pass (z-ai/glm-5.2)audited 2026-07-31
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  • 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.
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Every paracompact Hausdorff space is regular

Statement

Every paracompact Hausdorff topological space is regular. No choice principle is used.

Facts & Assumptions

Given: A paracompact Hausdorff space X, a closed set F⊆X, and a point p∈X∖F.

[F2]

A paracompact space gives every open cover a locally finite open refining cover (Paracompactness: every open cover has a locally finite open refinement, with no separation axiom built into the word).

[F3]

Regularity is separation of a point from a disjoint closed set by disjoint open sets (Regular spaces and T3 spaces, with the source disagreement over whether regularity includes T1 stated explicitly).

Proof

technique · direct
1.1

For every x∈F, Hausdorffness gives disjoint open sets U,V with x∈U and p∈V; hence p∉U‾, since X∖V is closed and contains U. Thus the family of all open U with U∩F≠∅ and p∉U‾, together with X∖F, is an open cover U of X.

F1construct
2.1

Take a locally finite open cover W refining U, and put H:=⋃{W∈W:W∩F≠∅}.

F2step 1.1chooseconstruct
3.1

The set H is open and contains F: a member of W containing a point of F cannot refine X∖F, so it occurs in the defining union.

step 1.1step 2.1
3.2

Every W occurring in H lies in an eligible U of step 1.1, so p∉W‾; local finiteness and [L1] give H‾=⋃W‾, whence p∉H‾.

step 1.1step 2.1L1
4.1

The open sets X∖H‾ and H contain p and F respectively and are disjoint. By [F3], X is regular.

step 3.1step 3.2F3∎

Depends on

Used by

Dependency tree · two levels

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Sources