Past exam of the mathematics course of the University of Cambridge / 2010 / ia / Paper 2 / 11F / c / Solution

Summing the geometric series for the offspring probability generating function givesThe fixed-point equation $s=1/(2-s)$ is $(s-1{)}^2=0$, whose only root in $[0,1]$ is one. The Galton-Watson extinction fixed point result therefore proves extinction is certain.

For the explicit iterates, the proposed formula at $n=1$ is $1/(2-s)$, so the base case holds. If it holds at $n$, part (a) givesThis is exactly the same formula with $n$ replaced by $n+1$. By induction,Evaluating the probability generating function at zero selects the mass of an empty generation:These probabilities increase to one, also confirming the extinction conclusion directly.