ThmDex – An index of mathematical definitions, results, and conjectures.
Formulation F10017 on D16: Countable set
F10017
Formulation 1
Let $\mathbb{N}$ be the D225: Set of natural numbers.
A D11: Set $X$ is a countable set if and only if there exists a D18: Map $f : E \to X$ such that
(1) $E \subseteq \mathbb{N}$ is a D78: Subset of $\mathbb{N}$
(2) $f$ is a D468: Bijective map from $E$ to $X$