\( \newcommand{\cat}[1] {\mathrm{#1}} \newcommand{\catobj}[1] {\operatorname{Obj}(\mathrm{#1})} \newcommand{\cathom}[1] {\operatorname{Hom}_{\cat{#1}}} \)

Equivalent formulations of function continuity

Let \( X \) be a subset of \( \mathbb{R} \), let \( f : X \to \mathbb{R} \) be a function, let \( x_0 \) be an element of \( X \). Then the following four statements are logically equivalent:

1. \( f \) is continuous at \( x_0 \).
2. For every sequence \( (a_n)_{n=0}^{\infty} \) consisting of elements of \( X \) which converges to \( x_0 \), the sequence \( (f(a_n))_{n=0}^{\infty} \) converges to \( f(x_0) \).
3. [...]
4. [...]