Definition
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[2DF]Recall that a set is ”countably infinite” if it has the same cardinality of \(ℕ\).
If \(A\) is countably infinite, there exists \(a:ℕ→ A\) bijective. Writing \(a_ n\) instead of \(a(n)\), we will therefore say that \(A=\{ a_{0},a_{1},a_ 2\ldots \} \) is an enumeration.