# Math Help - preimage and image stuff

1. ## 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

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.