EDB — 2DF

[2DF]Recall that a set is ”countably infinite” if it has the same cardinality of $\mathbb{N}$.

If $A$ is countably infinite, there exists $a:\mathbb{N}\to A$ bijective. Writing $a_n$ instead of $a(n)$, we will therefore say that $A=\{a_0,a_1,a_2\dots \}$ is an enumeration.