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February 13th, 2008, 06:55 PM  #1 
Newbie Joined: Feb 2008 Posts: 4 Thanks: 0  An error on functions and powersets, in a Dover book?
Hey all, Taking discrete mathematics this semester and I'm really enjoying the course material. I'm digging a bit farther into it because I have an interest in these topics, and, of course, I want to get an A in the class. So I'm working on a proof provided in the Dover book "Elements of Abstract Algebra" by Allan Clark. In it, there is a theorem which states: There is no onetoonecorrespondence f: X > 2^X for any set X. (Note that 2^X in this context refers to the powerset of X) For the vast majority of sets, this is true.. But what about the empty set ({}) ? If I'm not mistaken 2^{} = {{}} and a mapping: {} > {{}} Is both onetoone and onto. So, is this an error in the book? Or am I doing something wrong? Although the book provided a proof by contradiction, I gave it a try without. I'm still learning proofs, so I'd like advice if it's wrong, or could be improved: Theorem: There is no onetoone correspondence f: X > 2^X for any set X Let n(X) := the number of elements in a set X In order for f to have onetoone correspondence, the Domain(f) and Codomain(f) must have equal number of elements. However, for any set X, the 2^X has 2^n(X) elements, therefore f: X> 2^X cannot be a onetoone correspondence. That was my first try. If the theorem was changed to not include the empty set, it would be correct.. if I'm not already mistaken.  Rob Update: Correction, thanks to CRGreathouse 
February 13th, 2008, 07:00 PM  #2 
Global Moderator Joined: Nov 2006 From: UTC 5 Posts: 16,046 Thanks: 937 Math Focus: Number theory, computational mathematics, combinatorics, FOM, symbolic logic, TCS, algorithms 
Ah, but you did make a mistake early on. You wrote that 2^{} = {}, but in fact the power set of {} is not {} but {{}}, a set which has one element instead of the zero elements in {}. Your proof works, though, and does tell you that the power set of a 0element set will have 1 element. You almost figured it out... just like Saccheri nearly discovered nonEuclidean geometry. :P 
February 15th, 2008, 05:22 PM  #3 
Member Joined: Feb 2008 Posts: 89 Thanks: 0  Greetings: In order that a 11 correspondence may exist between two sets, they must both share the same cardinality. But for any set, S, n(P(S)) = 2^n(S), where P(S)==pwr set of S, and n(Si) refers to the cardinality of set Si. That is, the cardinality of P(S) is equal to 2 raised to the cardinality of S. Because 2^x > x for all nonnegative integers, x, it is not possible for S and P(S) to have like cardinality. Hence no such 11 correspondence exists and the proof is therefore complete. Regards, Rich B. 

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book, dover, error, functions, powersets 
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