Calculate a Feigenbaum's constant by period doubling obtained from Fourier transform of the output signal

Hi,

I have as a homework this task : "Calculate a Feigenbaum's constant by period doubling obtained from Fourier transform of the output signal". I have my signal in variable sol.y, and I succeeded to Fourier Transform it in the last line. Now I fail to relate Feigenbaum's constant to fourier transform. So I have my FFT now internet fails to mention how to obtain the constant now. My signal by the way, is just a sin wave.

c = 0;
b = 0;
a = 0;
d = -1;
w=1.3;
syms y(t)
[V] = odeToVectorField(diff(y, 2) == d*y - c*y^3-b*diff(y) + a*sin(w*t));
M = matlabFunction(V,'vars', {'t','Y'});
tspan = [0 100]  %for the span of 600 to 900 does not GIVE A PLOT
y0 = [1 1];
sol = ode45(M, tspan, y0);
figure (1)
fplot(@(x)deval(sol,x, 1), tspan);
title('position vs time');
xlabel('time t');
ylabel ('position x');
fourier = fft(sol.y)