- #1

member 428835

I am trying to replicate the phase portrait at the bottom right of this page: https://en.wikipedia.org/wiki/Duffing_equation

What I have is this:

Code:

```
global delta alpha beta gamma OMEG
delta = 0.3; % DAMPING
alpha = -1; % STIFFNESS
beta = 1; % RESTORATION NONLINEARITY
OMEG = 1.2; % DRIVING FORCE ANGULAR FREQ
gamma = 0.2; % FORCING
Fs = 100; % Sampling frequency
T = 1/Fs; % Sampling period
L = 10000; % Length of signal
[t, x] = ode45(@duffing,(0:L-1)*T,[0 1]);
[~, idx] = min( abs( t-OMEG/(2*pi)*38 ) );
plot(x(1:idx,2),x(1:idx,1))
title('phase space')
xlabel('position')
ylabel('velocity')
```

and the function ode45 calls is

Code:

```
function xdot = duffing(t,x)
global delta alpha beta gamma OMEG
xdot(1) = -delta*x(1) - alpha*x(2) - beta*x(2)^3 + gamma*cos(OMEG*t);
xdot(2) = x(1);
xdot = xdot';
end
```

For these parameter values I should get a cyclic phase portrait, but I don't. Can you see why?