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For compact Hausdorff spaces, total disconnectedness and total separatedness are equivalent
Statement
For compact Hausdorff spaces, total disconnectedness and total separatedness are equivalent.
Facts & Assumptions
Given: A compact Hausdorff space .
In a compact Hausdorff space, quasicomponents and connected components coincide (In a compact Hausdorff space every quasicomponent is connected, so quasicomponents and components coincide).
The notions of compactness and Hausdorffness are those of Open cover, subcover, and compact topological space; a compact subset is a subspace that is compact in its own right and Hausdorff space: distinct points have disjoint open neighbourhoods; every metrizable space is Hausdorff and the indiscrete topology on two points is not.
Total disconnectedness and total separatedness are the two notions fixed in Totally disconnected spaces and totally separated spaces.
Proof
Assume first that is totally separated. Let be connected and choose distinct points if possible. By [L2] there is a clopen set with and . Then and are disjoint nonempty clopen subsets of , contradicting connectedness. Hence every connected component is a singleton, so is totally disconnected.
Assume now that is totally disconnected. By [L1], quasicomponents also are singletons. For distinct points , the definition of quasicomponent therefore supplies a clopen set containing but not . This is exactly total separatedness by [L2].
Steps 1.1 and 1.2 prove the equivalence for compact Hausdorff spaces.
Depends on
- Totally disconnected spaces and totally separated spaces
- In a compact Hausdorff space every quasicomponent is connected, so quasicomponents and components coincide
- Open cover, subcover, and compact topological space; a compact subset is a subspace that is compact in its own right
- Hausdorff space: distinct points have disjoint open neighbourhoods; every metrizable space is Hausdorff and the indiscrete topology on two points is not
Used by
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Sources
- Brian Osserman, Math 6112 notes on inverse limits and profinite groups (standard reference, not scraped)
- H. W. Lenstra, Profinite groups and Galois groups (standard reference, not scraped)