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1. The Baire category theorem - Aarhus Universitet

1. The Baire category theorem - Aarhus Universitet

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On locally compact Hausdorff spaces<br />

Lemma<br />

Let X be a Hausdorff space and {Kα : α ∈ A} a collection of<br />

compact subsets of X. If <br />

α∈A Kα = ∅, there is a finite subset<br />

F ⊆ A such that <br />

α∈F Kα = ∅.<br />

Proof.<br />

<br />

Fix one of the Kα’s; say Kα1 . Since α∈A Kα = ∅, every element of<br />

Kα1 is in the complement Kc α of Kα, for some α. This means that<br />

{Kc α : α ∈ A} covers Kα1 .<br />

Klaus Thomsen <strong>1.</strong> <strong>The</strong> <strong>Baire</strong> <strong>category</strong> <strong>theorem</strong>

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