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, Set Theory, Natural number, Introducing Power Sets, cardinality result

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