Saturday, 16 April 2016

elementary set theory - Need help for proving that: f(f1(A))A

I realised I needed to show more information, which I now did:




f:XXandAX



Proof that: f(f1(A))A




This is my proof:




By defintion:



f1(A)={xXf(x)A}
and



f(A)={f(x)xA}={yXxA:y=f(x)}X



Therefore we can end the proof by a final definition:\
f(f1(A))={yA:xf1(A):y=f(x)}A




Is this a legit "proof"? And is it even a proof, when i only use definitions?

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