# preimage and image stuff

• Nov 17th 2009, 05:28 PM
preimage and image stuff
how do you prove the preimage of the image of the domain is equal to the domain and how do you prove the image of the preimage of the codomain is the codomain
• Nov 17th 2009, 07:09 PM
• Nov 17th 2009, 07:57 PM
Drexel28
Quote:

Problem: Let $\phi:X\mapsto Y$. Prove that $X=\phi^{-1}\left[\phi\left(X\right)\right]$
Proof: Let $x\in\phi^{-1}\left[\phi\left(X\right)\right]$. Then $\phi(x)\in \phi\left(X\right)\implies x\in X$. Therefore $\phi^{-1}\left[\phi\left(X\right)\right]\subset X$. Now let $x\in X$, $\phi(x)\in\phi\left(X\right)$. Which means that $x\in\phi^{-1}\left[\phi\left(X\right)\right]$. Therefore $X\subset\phi^{-1}\left[\phi\left(X\right)\right]$. The conclusion follows.