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Under choice, every open cover of a paracompact Hausdorff space has locally finite open refinements and with
Statement
Assume the Axiom of Choice. If is paracompact and Hausdorff and is an open cover, there are a set , a map from into , and locally finite open covers and with
Facts & Assumptions
Given: The Axiom of Choice, a paracompact Hausdorff space , and an open cover .
Every family of nonempty sets has a choice function (The Axiom of Choice).
The space is regular (Every paracompact Hausdorff space is regular).
In a regular space, open gives an open with (A space is regular if and only if every point has a neighbourhood base of closed neighbourhoods, if and only if open gives an open with , implication (a)(b)).
Every open cover has a locally finite open refinement (Paracompactness: every open cover has a locally finite open refinement, with no separation axiom built into the word).
Locally finite unions commute with closure (Locally finite families remain locally finite after taking closures, closure commutes with their union, and a locally finite union of closed sets is closed).
Proof
We first prove a one-shrink construction for any open cover . Let be the family of all open for which for some . By [L1] and [L3], covers . Take a locally finite open refining cover of by [F1], discard its empty members, and use [A1] to assign to each sets and with Then .
Apply step 1.1 to . This gives a locally finite open cover and assigned such that .
Apply step 1.1 again, now to the cover . Obtain a locally finite open cover and a map such that . For put The family is an open cover. It is locally finite because any neighbourhood meeting only finitely many meets only the corresponding finitely many grouped unions .
Each subfamily is locally finite, so [L2] gives Together with step 2.1 this yields for every , with both displayed families locally finite open covers.
Remarks
The Axiom of Choice is used to retain the assignments to cover members through the two locally finite refinements. This is a sufficient hypothesis for this construction; no claim is made that it is the exact choice strength.
Depends on
- Every paracompact Hausdorff space is regular
- A space is regular if and only if every point has a neighbourhood base of closed neighbourhoods, if and only if $x \in U$ open gives an open $V$ with $x \in V \subseteq \overline{V} \subseteq U$
- Locally finite families remain locally finite after taking closures, closure commutes with their union, and a locally finite union of closed sets is closed
- Paracompactness: every open cover has a locally finite open refinement, with no separation axiom built into the word
- The Axiom of Choice
Used by
Dependency tree · next 3 levels
Direct dependencies and their dependencies through the next three levels: 28 results over 9 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
- Dartmouth Point-Set Topology, Lecture 25 (standard reference, not scraped)
- Topology 262 notes (California State University, Northridge) (standard reference, not scraped)
- R. Gardner, Notes on Munkres Section 41: Paracompactness (East Tennessee State University) (standard reference, not scraped)