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Unformatted text preview: HW 3 1 1. (a) Let ( A; & ) be a wellordered set, B ¡ A and f : A onto ! B be an order isomorphism of ( A; & ) onto ( B; & ) . Prove that for every a 2 A , a & f ( a ) . (b) Let ( A; & ) be a wellordered set and a 2 A . Recall that the set I a = f x 2 A j x ¢ a g is called an initial segment of A . Use part (a) to prove that ( A; & ) cannot be order isomorphic to one of its initial segments. Proof. (a) Contrary to the claim, suppose that there is a 2 A such that f ( a ) ¢ a . Then the set A = f a 2 A j f ( a ) ¢ a g is not empty because a 2 A . Because ( A; & ) is a wellordered set and A 6 = ? , the set A contains the smallest element a 2 A , i.e., a & a 8 a 2 A : Now set b = f ( a ) . Because b = f ( a ) ¢ a and because f preserves the order, we have f ( b ) ¢ f ( a ) = b , i.e., b 2 A and b ¢ a , a contradiction because a is the smallest element of A . (b) Contrary to the claim, suppose that there is a 2 A such that ( A; & ) is order isomorphic to the initial segment I a . Then 8 x 2 A f ( x ) ¢ a; in particular, f ( a ) ¢ a , a contradiction with the result of part (a). 2. An ideal in a partial ordered set ( X; & ) is a set I ¡ X such that ( x 2 I ) ^ [( y 2 X ) ^ ( y & x )] ) y 2 I . Prove that every ideal I in a wellordered set ( X; & ) is either an initial segment of ( X; & ) or X . Proof. If I = X , then we are done. Let I 6 = X . Let X n I 6 = ? . Set b = inf ( X n I ) . We claim that I = I b . Indeed, if x 2 I , then x & b ; 1 Note to the grader. Please grade the following problems: #1, 2, #3, 4, 5 (20 pts for each problem) 1 otherwise, b & x . Because I is an ideal, b 2 I , a contradiction. So, I ¡ I b . Now we show that I b n I = ? . Contrary to the claim, suppose that there is y 2 I b n I . Then y & b because y 2 I b and y < b because y 2 X n I and b is the smallest element in X n A . Hence, because y & b and y < b , we arrive at a contradiction. So, I...
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This note was uploaded on 02/16/2012 for the course MATH 540 taught by Professor Ruan during the Spring '08 term at University of Illinois, Urbana Champaign.
 Spring '08
 Ruan

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