20) prove that p ia an accumulation point of S if and only if every ball B about p intersects S - {p}
21) prove or give a counterexample : there are only countably many sequences with limit 0.
22) prove or give a counterexample : a real increasing sequence
a1 less than a2 less than a3....................... converges if and only if the differences an+1-an converge to 0.