Tuesday, 20 December 2016

elementary set theory - Is this proof correct for : Does F(A)capF(B)subseteqF(AcapB) for all functions F?



Is this proof correct? To prove F(A)F(B)F(AB) for all functions F.




Let any number yF(A)F(B). We want to show yF(AB).




Therefore, yF(A) and yF(B), by definition of intersection.



By definition of inverse, y=F(x) for some xA and xB



And so, y=F(x) for some xAB



And therefore, yF(AB)





I have a gut feeling deep down that something is wrong. Can anyone help me pinpoint the mistake? I am not sure why am I having so much problems with functions. Am I not thinking in the right direction?



Sources : 2nd Ed, P219 9.60 = 3rd Ed, P235 9.12, 9.29 - Mathematical Proofs, by Gary Chartrand,
P214 Theorem 12.4 - Book of Proof, by Richard Hammack,
P257-258 - How to Prove It, by D Velleman.


Answer



The third line is mistaken. You only know that there exists an x in A such that F(x)=y, and you know there is a zB such that F(z)=y.



It is extremely easy to find a counterexample: just draw two sets A, B that are disjoint, and map an aA and a bB to a single point. Then you have that yF(A)F(B), but F(AB)=.


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