Is it possible to have a map $f:X\to Y$ from a topological space $X$ to a set $Y$ and some subsets of $Y$ namely $U_i,i\in I$ such that $\bigcup_{i\in I} f^{-1}(U_i)$ is not equal to $f^{-1}\left(\bigcup_{i\in I}U_i\right)$ ?
I can't think of a case that this is true, but of course this doesn't mean anything! Thanks.