A discrete random variable X has a probability mass function given by

px (x) = (k(p^x))/x for x=1,2...

where p is a constant such that 0 < p < 1 and k = -1/ln(q) where q = 1-p

a) Show that the probability generating function of X is given by

E(t^x) = (ln(1 - pt))/ln(q) for |t| < 1/p

b) Why must t be restricted to this range?

any help is greatly appreciated!