Emergence of phase concentration for the Kuramoto-Sakaguchi equation

We study the asymptotic phase concentration phenomena for the Kuramoto-Sakaguchi (K-S) equation in a large coupling strength regime. The main improvement of this work with respect to the literature is that the confinement assumption of the initial phase configuration is removed. For this, we analyze the detailed dynamics of the order parameters such as the amplitude and the average phase. For the infinite ensemble of oscillators with the identical natural frequency, we show that the total mass distribution concentrates on the average phase asymptotically, whereas the mass around the antipodal point of the average phase decays to zero exponentially fast in any positive coupling strength regime. Thus, generic initial kinetic densities evolve toward the Dirac measure concentrated on the average phase. In contrast, for the infinite ensemble with distributed natural frequencies, we find a certain time-dependent interval whose length can be explicitly quantified in terms of the coupling strength. Provided that the coupling strength is sufficiently large, the mass on such an interval is eventually non-decreasing over the time. We also show that the amplitude order parameter has a positive lower bound that depends on the size of support of the distribution function for the natural frequencies and the coupling strength. The proposed asymptotic lower bound on the order parameter tends to unity, as the coupling strength increases to infinity. This is reminiscent of practical synchronization for the Kuramoto model, in which the diameter of the phase configuration is inversely proportional to the coupling strength. The estimate of the lower bound on the order parameter yields the asymptotic mass transportation into the attained interval, which consequently leads to the synchronization. Our results for the K-S equation generalize the results in Ha et al. (2016) on the emergence of phase-locked states for the Kuramoto model in a large coupling strength regime. (C) 2019 Published by Elsevier B.V.