▾ Set of symbols
▾ Alphabet
▾ Deduction system
▾ Theory
▾ Zermelo-Fraenkel set theory
▾ Set
▾ Binary cartesian set product
▾ Binary relation
▾ Map
Countable map
Formulation 0
A D18: Map $x : J \to X$ is countable if and only if $J$ is a D16: Countable set.