From your own link on countable sets: > Equivalently, a set S is countable if there exists an injective function f : S → N from S to N; it simply means that ev

From your own link on countable sets: > Equivalently, a set S is countable if there exists an injective function f : S → N from S to N; it simply means that every element in S corresponds to a different element in N. Defining N is usually done via a successor set, on which case 0 makes no sense to include.