Past exam of the probability, statistics and options I course of the University of Oxford / 2008 / E / 1 / b / Solution

A basis of a topology is a collection $\mathcal{B}$ of open sets such that every open set is a union of members of $\mathcal{B}$. Equivalently, for every open $U$ and $y\in U$ there is $B\in \mathcal{B}$ with $y\in B\subseteq U$.

A map $f:X\to Y$ is continuous exactly when $f^{-1}(B)$ is open in $X$ for every basic set $B$ of $Y$. Necessity follows from the definition of a continuous map. For sufficiency, express an arbitrary open set as a union of basic sets and use preservation of unions by inverse images.