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LemmaStatement: Literature-sourcedProof: AI-adaptedSession-authored (Fable 5 assisted)precheck passjudge pass (z-ai/glm-5.2)audited 2026-07-31
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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 XX, a closed set FXF\subseteq X, and a point pXFp\in X\setminus 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 T3T_3 spaces, with the source disagreement over whether regularity includes T1T_1 stated explicitly).

Proof

technique · direct
1.1

For every xFx\in F, Hausdorffness gives disjoint open sets U,VU,V with xUx\in U and pVp\in V; hence pUp\notin\overline U, since XVX\setminus V is closed and contains UU. Thus the family of all open UU with UFU\cap F\ne\varnothing and pUp\notin\overline U, together with XFX\setminus F, is an open cover U\mathcal U of XX.

F1construct
2.1

Take a locally finite open cover W\mathcal W refining U\mathcal U, and put H:={WW:WF}H:=\bigcup\{W\in\mathcal W:W\cap F\ne\varnothing\}.

F2step 1.1chooseconstruct
3.1

The set HH is open and contains FF: a member of W\mathcal W containing a point of FF cannot refine XFX\setminus F, so it occurs in the defining union.

step 1.1step 2.1
3.2

Every WW occurring in HH lies in an eligible UU of step 1.1, so pWp\notin\overline W; local finiteness and [L1] give H=W\overline H=\bigcup\overline W, whence pHp\notin\overline H.

step 1.1step 2.1L1
4.1

The open sets XHX\setminus\overline H and HH contain pp and FF respectively and are disjoint. By [F3], XX is regular.

step 3.1step 3.2F3

Depends on

Used by

Dependency tree · next 3 levels

Direct dependencies and their dependencies through the next three levels: 52 results over 13 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