  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.
