Synchronization of Coupled Anizochronous Auto-Oscillating Systems
  The particular properties of synchronization are discussed for coupled auto-oscillating systems, which are characterized by non-quadratic law of potential dependence on the coordinate. In particular, structure of the parameter plane (frequency mismatch - coupling value) is considered for coupled van der Pol - Duffing oscillators. The arrangement of synchronization tongues and the particular properties of their internal structure in the parameter space are revealed. The features of attractors in the phase space are discussed. 
A.P. Kuznetsov, Ju.P. Roman