Find a closed form for the generating function for the sequence {a_n}, where a) a_n = -1 for all n = 0, 1, 2, ... b) a_n = 2^n for n = 1, 2, 3, 4, ... and a_0 = 0. c) a_n = n - 1 for n = 0, 1, 2, ... d) a_n = 1/(n + 1)! for n = 0, 1, 2, .... e) a_2 = (n/2) for n = 0, 1, 2