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A nonempty compact Hausdorff space carries exactly one compatible uniformity
Statement
A nonempty compact Hausdorff topology carries exactly one compatible uniformity.
Facts & Assumptions
Given: A nonempty compact Hausdorff topology on .
Its open covers form a compatible uniform-cover structure (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).
Uniform-cover and entourage structures determine each other (On a nonempty set, entourage uniformities and uniform-cover structures determine one another).
A compatible uniformity is one whose induced topology is the given topology (Uniformizable and separated-uniformizable topological spaces).
Entourage balls form neighbourhood bases, symmetric entourages have square roots, and compactness supplies finite subcovers (The sets containing an entourage ball about each of their points form a topology, Every uniformity has a base of symmetric entourages, Open cover, subcover, and compact topological space; a compact subset is a subspace that is compact in its own right).
Every open cover of a compact Hausdorff space has a finite open star-refinement (Every open cover of a compact Hausdorff space has a finite open star-refinement).
Proof
Apply [L2] to the cover structure of [L1] to obtain one compatible entourage uniformity.
Let be any compatible uniformity. Each entourage-ball cover admits an open refinement because every ball is a neighbourhood in the induced topology, so every cover uniform for admits an open refinement.
Conversely, let be an open cover and take a finite open star-refinement by [L5]. Form the family of all open sets for which there are , , and a symmetric entourage satisfying and . This family covers : given , first take containing it, then use compatibility and a symmetric square root to obtain such and an open neighbourhood . Compactness gives finitely many witnesses covering . Put . If and , then symmetry gives , so . Hence the -ball cover refines , and therefore refines . Thus every open cover is uniform for .
By steps 1.2 and 1.3, the cover structure associated to consists exactly of the covers admitting an open refinement, which is the structure in [L1].
The dictionary [L2] then recovers the same entourage uniformity from either structure, proving uniqueness.
Depends on
- 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
- On a nonempty set, entourage uniformities and uniform-cover structures determine one another
- Uniform space in the entourage formulation
- Every open cover of a compact Hausdorff space has a finite open star-refinement
- Every uniformity has a base of symmetric entourages
- The sets containing an entourage ball about each of their points form a topology
- Open cover, subcover, and compact topological space; a compact subset is a subspace that is compact in its own right
- Uniformizable and separated-uniformizable topological spaces
Used by
- Every continuous map from a nonempty compact Hausdorff space to a uniform space is uniformly continuous Corollary
- Assuming the Ultrafilter Lemma and Countable Choice, an uncountable Cantor cube is compact Hausdorff and uniformizable but not first countable, hence not metrizable Example
- The closed unit interval has exactly one compatible uniformity, namely its usual metric uniformity Example
- Under dependent choice and the ultrafilter lemma, the Samuel compactification of a nonempty compact Hausdorff space adds no points up to unique uniform isomorphism Example
- The Samuel uniformity is totally bounded Lemma
- Under dependent choice and the ultrafilter lemma, uniformly continuous maps to compact Hausdorff spaces extend uniquely over the Samuel compactification Theorem
Dependency tree · next 3 levels
Direct dependencies and their dependencies through the next three levels: 59 results over 16 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
- J. Wodzicki, Uniform Structure (standard reference, not scraped)
- M. Kunzinger, General Topology (standard reference, not scraped)