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Unformatted text preview: = 3   = N = undeFned for now Fact Let A and B be Fnite sets. Then  A B  =  A  +  B  A B  . Informal proof The number  A  +  B  counts the elements of A B twice, so we subtract A B to obtain the result. A consequence of this proposition is that, if A and B are disjoint sets, then  A B  =  A  +  B  . 2.2.4 Introducing Power Sets In deFnition 2.2, we deFned a notion of a subset of a set. The set of all subsets of A is called the power set 3 of A . D EFINITION 2.9 (P OWER SET ) Let A be any set. Then the power set of A , written P ( A ) , is { X : X A } 3 The name might be due to the cardinality result in proposition 2.10. At least this explanation helps to remember the result! 10...
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 Spring '09
 Koskesh
 Math

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