Solution (source code)

= Solution

For every $\varepsilon>0$ there must be a $\delta>0$ such that
$$
\operatorname{dist}_X(x,x_0)<\delta\quad\Longrightarrow\quad
\operatorname{dist}_Y(f(x),f(x_0))<\varepsilon.
$$
The choice of $\delta$ may depend on $x_0$ and $\varepsilon$. This is the <epsilon-delta definition of continuity>.