Computing the population given a set of replication rates So I'm looking to calculate the probability that a bacterial population dies out with the following conditions:


*

*Initial population is 1

*At each iteration the bacteria can die, do nothing, divide into 2 or divide into three.  So each bacteria can change into either 0, 1, 2, 3 bacteria.

*Each event has a 25% likelihood.


So I know from simulation that the likelihood of the population dying out is ~36.3%. However, I'm looking for a method to calculate this analytically.
Some MATLAB code for the simulation:
ntries = 100000;
mnum = 10000;
res = ones(ntries, 1);


for i = 1:ntries
    while res(i) > 0 && res(i) < mnum
        res(i) = sum(randi(4,res(i), 1)-1);
    end
end
mean(res==0)

Thanks,
Will

Call $q_n$ the probability that the initial bacteria dies out at or before generation $n$ and condition by the size $i$ of the first generation. The event that every bacteria of this generation dies out at or before generation $n+1$ has probability $q_n^i$ and $i=0$, $1$, $2$ or $3$ with probability $1/4$, hence $q_{n+1}=g(q_n)$ with 
$$
g(q)=q^0/4+q^1/4+q^2/4+q^3/4.
$$
Since $q_0=0$, $q_n=g\circ g\circ\cdots\circ g(0)$ ($g$ composed $n$ times). One can check, for instance by considering the graph of $g$ on the interval $(0,1)$, that $q_n$ converges to the smallest nonnegative root $q$ of the equation $q=g(q)$.
Now $g(q)=q$ if and only if $0=q^3+q^2-3q+1=(q-1)(q^2+2q-1)$, hence $q=1$ or $(q+1)^2=2$, that is $q=-1\pm\sqrt{2}$. Finally the population dies out with probability $q=\sqrt2-1\approx41\%$.
See this chapter of lecture notes by Steve Lalley, section 1.2.
