Solution
= Solution
The <function> $f$ is <continuous> at $x_0$ if
$$
\boxed{\forall\varepsilon>0\ \exists\delta>0\ \forall x\in\mathbb R:
\quad |x-x_0|<\delta\ \Longrightarrow\ |f(x)-f(x_0)|<\varepsilon}.
$$
The choice of $\delta$ may depend on $\varepsilon$ and $x_0$, but must work for every $x$ in that neighbourhood. This is the <epsilon-delta definition of continuity>.