function rerr = relreserr(orbit,T)
[m,n] = size(orbit);
orbitf = fft(orbit.').';
orbitf = orbitf.* (ones(3,1)*sqrt(-1)*[0,1:n/2-1,0,-n/2+1:-1]);
dorbit = real(ifft(orbitf.').');
w = 2*pi/T;
err = w*dorbit - vfield(orbit);
rerr = norm(err, 'fro')/ (w*norm(dorbit, 'fro'));
