N1=100; N2=400;
a=(2.5:0.01:4)';
m=length(a);
x(1:m,1)=1/2;
for i=1:N2
  x(:,i+1)=a.*x(:,i)-a.*x(:,i).^2;
end;
plot(a,x(:,N1:N2),'.k'); xlabel('a'); 