Suppose I have a countable set B, and a subset of it called A. I would like to know when is A finite.
From the comments: B is an infinite set. I know that B is equivalent to the natural numbers so every subset of B is either countable (when there exists a bijection between A and B) or A is a finite set (when? - Does it happens if there is no bijection from A to B?) – JanosAudron 10 mins ago