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TheoremStatement: AI-adaptedProof: AI-adaptedprecheck passverified 2026-08-09 (gpt-5.6-terra-codex-subscription)
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The covers admitting an open refinement form a compatible uniform-cover structure on a nonempty compact Hausdorff space; in particular every open cover is uniform

Statement

For a nonempty compact Hausdorff space, the covers that admit an open refinement form a compatible uniform-cover structure. In particular every open cover is uniform.

Facts & Assumptions

Given: A nonempty compact Hausdorff space X.

[L1]

Every open cover has a finite open star-refinement (Every open cover of a compact Hausdorff space has a finite open star-refinement).

[L2]

A uniform-cover structure is closed under coarsening and common refinement and has star-refinements (Uniform space in the uniform-cover formulation).

[L3]

A uniform-cover structure determines an entourage uniformity with basic relations EV=⋃V∈VV×V, and entourage balls form neighbourhood bases for the induced topology (On a nonempty set, entourage uniformities and uniform-cover structures determine one another, The sets containing an entourage ball about each of their points form a topology).

Proof

technique · direct
1.1

Let C be the covers admitting an open refinement. It is nonempty, since {X} is open.

construct
2.1

Coarsening preserves membership in C, and two open refinements have their intersection cover as a common open refinement.

step 1.1
2.2

For V∈C, take an open refinement and then its finite open star-refinement from [L1]; this is a star-refinement still witnessing membership in C.

L1step 1.1
3.1

Thus C satisfies [L2]. Since every open cover refines itself, every open cover belongs to C.

step 2.1step 2.2L2
4.1

The entourage uniformity recovered from C by [L3] induces the original topology. If V∈C, choose an open refinement W; then EW[x]=St⁡(x,W) contains an open member through x, so the recovered entourage balls are neighbourhoods in the original topology. Conversely, if x∈O with O open, the open cover {O,X∖{x}} belongs to C, since Hausdorffness makes {x} closed. By [L1] choose a finite open star-refinement W, which belongs to C, and choose W0∈W containing x. The star of W0 lies in O, rather than in X∖{x}, and therefore EW[x]=St⁡(x,W)⊆St⁡(W0,W)⊆O.

step 3.1L1L3
5.1

Hence the structure is compatible with the given topology, and every open cover is uniform.

step 3.1step 4.1∎

Depends on

Used by

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