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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Basic properties the notions of <strong>category</strong><br />
(a) If A ⊆ B and B is of the first <strong>category</strong>, then so is A.<br />
Indeed if B = ∞<br />
i=1 Bi, then A = ∞<br />
i=1 Bi ∩ A and Bi ∩ A ⊆ Bi.<br />
(b) Any countable union of sets of the first <strong>category</strong> is of the first<br />
<strong>category</strong>.<br />
Indeed, if B = ∞ i=1 Bi and Bi = ∞ j=1 B(i,j), for all i, then<br />
B(i,j), i,j ∈ N is a countable collection of sets whose union is B.<br />
(c) Any closed set with empty interior is of the first <strong>category</strong>.<br />
Indeed, if E is closed, E = E ∪ ∅ ∪ ∅ ∪ ∅ ∪ ... is a union of a<br />
countable collection of nowhere dense sets.<br />
Klaus Thomsen <strong>1.</strong> <strong>The</strong> <strong>Baire</strong> <strong>category</strong> <strong>theorem</strong>