Phase-locked and phase drift solutions of phase oscillators with   asymmetric coupling strengths
  Phase-locked solutions of coupled oscillators are studied with asymmetric coupling strengths or inhomogeneous natural frequencies. The solutions show remarkable profiles of phase lags from the pacemaker corresponding to the ratio of upward and downward coupling strengths. By means of the existence condition of phase-locked solutions, the transition points from phase-locked to phase drift states are estimated. The application of the existence condition to the case of the linear gradient of natural frequency illustrates some scaling properties in the frequency diagrams. 
H. Yamada