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Unformatted text preview: 3.3 Compact sets 55 Corollary 3.3.7. Any infinite subset of a compact set has a cluster point. Proof. This comes from the two previous theorems Definition 3.3.8. An open cover of A R is a collection of open sets { U i } with A S U i . Example 3.3.1. (0 , 1) (0 , 1) ( 1 , 2). (0 , 1) (0 , 2 3 ) ( 1 3 , 1). (0 , 1) S n ( 1 n , 1 1 n ). R S n ( n,n ). U S x U ( x ,x + ). Theorem 3.3.9. Let K R be compact, and let B be a closed subset of K . Then B is compact. Proof. K is closed and bounded, by thm, so B K must also be bounded. Since B is closed by hypothesis, B is compact. Theorem 3.3.10 (HeineBorel Thm) . A set is compact iff every open cover has a finite subcover. Proof. ( ) Suppose A is an open cover of a compact set K . First, reduce A to a countable subcover B . Let I be any open interval with rational endpoints. If there is an open set of A which contains I , then add it to B . If not, then dont. Now any point of K is contained...
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 Fall '11
 Wong
 Sets

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