Solution
= Solution
A map of <topological spaces> is <continuous> when the <preimage> $f^{-1}(U)$ is open in $X$ for every <open set> $U$ of $Y$. Equivalently, the <preimage> of every <closed subset> of $Y$ is closed in $X$.
= Solution
A map of <topological spaces> is <continuous> when the <preimage> $f^{-1}(U)$ is open in $X$ for every <open set> $U$ of $Y$. Equivalently, the <preimage> of every <closed subset> of $Y$ is closed in $X$.