I'm having trouble finding a proof (without the need for cases) for this statement: If A is at most countable, then there is a sequence $(a_n)_n$ such that $ A = \{a_n : n \in \Bbb N \} $.
I know that there exists a surjection from the naturals onto A, but can we then define that surjection as a sequence?
I know this should be simple. But I just don't know how to write a nice, technically sound proof.
Thanks