clear
rand('state', 100)
x = [1 2]
for n = 2: 999,
    x(n+1) = x(n) + sign(rand - 0.5)*x(n-1);
end
semilogy(1:1000, abs(x))
c=1.13198824
hold on
semilogy(1:1000, c.^[1:1000])
hold off