ThmDex – An index of mathematical definitions, results, and conjectures.
Formulation F10016 on D16: Countable set
F10016
Formulation 0
Let $\mathbb{N}$ be the D225: Set of natural numbers.
A D11: Set $X$ is a countable set if and only if \begin{equation} \exists \, E \subseteq \mathbb{N} : \text{Bij}(E \to X) \neq \emptyset \end{equation}