10.04.2013 Views

1. The Baire category theorem - Aarhus Universitet

1. The Baire category theorem - Aarhus Universitet

1. The Baire category theorem - Aarhus Universitet

SHOW MORE
SHOW LESS

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

On locally compact Hausdorff spaces<br />

Proof.<br />

Consider an element k ∈ K. Since X is Hausdorff there are open<br />

sets, Wk ⊆ X and Uk ⊆ X, such that k ∈ Wk,p ∈ Uk and<br />

Wk ∩ Uk = ∅.<br />

Note that the collection {Wk : k ∈ K} is an open cover of K.<br />

Since K is compact there is a finite collection k1,k2,... ,kN such<br />

that<br />

K ⊆ Wk1 ∪ Wk2 ∪ · · · ∪ WkN .<br />

Set W = Wk1 ∪ Wk2 ∪ · · · ∪ WkN and U = Uk1 ∩ Uk2 ∩ · · · ∩ UkN .<br />

Corollary: In a Hausdorff space, any compact subset is closed.<br />

Proof: By Lemma1 the complement of a compact set is open.<br />

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

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!