Question

prove that a subset of a countable set is ​

Answer 1

Answer:

Countability Lemma.

Suppose to each element of the set A there is assigned, by some definite rule, a unique natural number in such a manner that to each n∈ N there corresponds at most a finite number of elements of the set A. Then A is countable. ... If no such n exists, then S is called an infinite set.