Prove that a disjoint union of any finite set and any countably
infinite set is countably infinite.
Proof: Suppose A is any finite set,
B is any countably infinite set, and A and
B are disjoint. By definition of disjoint, A ∩ B = ∅
In case A = ∅, then A ∪ B = B, which
is countably infinite by hypothesis.
Now suppose A ≠ ∅. Then there is a positive integer
m so that A has m elements...