Fig. 1

Isochrons of a Stuart–Landau oscillator model: $\dot{x}=\lambda x/2-(\lambda c/2+\omega )y-\lambda (x^2+y^2)(x-cy)/2$, $\dot{y}=(\lambda c/2+\omega )x+\lambda y/2-\lambda (x^2+y^2)(cx+y)/2$. The black curve represents the periodic orbit of the system, which is simply the unit circle for this model. The blue curves are the isochrons obtained using the (forward) approach of Mauroy and Mezic [11]. The green dots are analytically obtained isochronal points [15]. Parameter values are $\lambda =2.0$, $c=1.0$, and $\omega =1.0$