Since this is supposed to be a property of n only (not of a, b), I assume the property is, "For all integers a and b, if n doesn't divide ab...". Let's denote it by P(n). Its contrapositive, which is equivalent to the original statement, is, "For all a and b, if n divides a or n divides b, then n divides ab". I agree that it is true for all n. To be formal, one can say that the necessary and sufficient condition Q(n) on n is True. I.e., we have for all n, Q(n) <-> P(n).