16. Give a recursive definition of (2 points each) (a) the set of odd positive integers (i.e., 1, 3, 5, 7, 9,...). (b) the set of positive integer powers of 4 (i.e. 4, 16, 64, 256, ...) (c) The set of integers f1, -5, 25, -125, 625, ...]