Let $f: X \to Y$ be a faithfully flat quasi-compact morphism. Then for a subset $V \subseteq Y$, $V$ is open in $Y$ iff $f^{-1}(V)$ is open in $X$?
I know this is EGA IV2 2.3.12. But its proof is very complicated for me. (I'm not familiar with pro-constructibility.) So are there other good proofs of it or its references?
Thank you very much!