(a) Time course x(t) of a van der Pol oscillator perturbed by external force F(t). The equation of motion is with μ = 0.3. Blue trace, perturbation F(t); black trace, x(t) before and after perturbation; green trace, x(t) during perturbation and dashed trace, continuation of unperturbed oscillation (perturbation not switched on, F(t) = 0). (b) Limit cycle of van der Pol oscillator. The black and green traces from (a) are replotted in the phase space (velocity dx/dt versus coordinate x); increasing time t corresponds to clockwise rotation.