Q4. Let Sn be the number of individuals in the n-th generation of a branching process (also called Galton-Watson process). Assume So = 1. Each of the Sn individuals in generation n independently produces a random number of offspring with common distribution for the (n + 1)-st generation. Let m := E[ok] be the mean number of offspring. Prove that the branching process dies out almost surely, i.e., almost surely Sn = 0 for all n sufficiently large, if m < 1 and (0) > 0. (Hint: Sn/mn is a martingale.)