function out = Xode(t,c)

global n2 N D a b

%  reconstruct u 

uf = [0 ; -flipud(c(2:end)) ; c];
u2 = ifft(fftshift(-i*uf))*N;
u  = real(u2(1:N));

%  calculate spectral forcings

f = (a - b*u.^2).*u;

f2 = [f ; 0 ; -flipud(f(2:end))];
ff = -imag(fftshift(fft(f2))/N);
cf  = ff(N+1:2*N);

out = -D*(n2.*c) + cf;