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1. The Baire category theorem - Aarhus Universitet

1. The Baire category theorem - Aarhus Universitet

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Proof of <strong>Baire</strong>’s <strong>category</strong> <strong>theorem</strong> - continued<br />

Let V1,V2,V3,... be open and dense subsets of S. We must show<br />

that ∞ i=1 Vi is dense in S when S is either a complete metric<br />

space or a locally compact Hausdorff space.<br />

Consider an open non-empty subset B0 of S. We want to show<br />

that ∞ i=1 Vi ∩ B0 = ∅. Since V1 is dense in S, B0 ∩ V1 = ∅.<br />

We claim that there is an open set B1 such that<br />

B1 ⊆ V1 ∩ B0<br />

When S is a complete metric space we will require that d(y,x) ≤ 1<br />

when x,y ∈ B1 and when S is locally compact Hausdorff we require<br />

that B1 is compact.<br />

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

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