On the conditions for synchronization in unidirectionally coupled   chaotic oscillators
  The conditions for synchronization in unidirectionally coupled chaotic oscillators are revisited. We demonstrate with typical examples that the conditional Lyapunov exponents (CLEs) play an important role in distinguishing between intermittent and permanent synchronizations, when the analytic conditions for chaos synchronization are not uniformly obeyed. We show that intermittent synchronization can occur when CLEs are very small positive or negative values close to zero while permanent synchronization occurs when CLEs take sufficiently large negative values. There is also strong evidence for the fact that for permanent synchronization the time of synchronization is relatively low while it is high for intermittent synchronization. 
P. Muruganandam, M. Lakshmanan, and S. Parthasarathy