1. The Baire category theorem - Aarhus Universitet
1. The Baire category theorem - Aarhus Universitet
1. The Baire category theorem - Aarhus Universitet
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Definition of first and second <strong>category</strong><br />
First <strong>category</strong><br />
A set A ⊆ S is of first <strong>category</strong> if it is the union of a countable<br />
collection of nowhere dense sets, i.e. if A = ∞ i=1 Ai, where Ai have<br />
empty interior.<br />
Example: <strong>The</strong> rational numbers Q form a set of first <strong>category</strong> in<br />
the real line R. Note that Q itself is not nowhere dense; but it is<br />
the union of a countable collection of sets that are!<br />
Klaus Thomsen <strong>1.</strong> <strong>The</strong> <strong>Baire</strong> <strong>category</strong> <strong>theorem</strong>