Give a recurrence relation for the population of Fibonacci's rabbits if the rabbits must be at least two months old to reproduce. The colony starts with one pair of newborn rabbits. Every pair of reproducing rabbits gives birth to a new pair of rabbits, one male and one female over the course of a month. Let gn denote the number of pairs of rabbits at the end of month n. Assume that the first pair of rabbits obtained for the colony are born at the end of month 1.