  A necessary condition for the emergence of chaos is given. It is well known
that the emergence of chaos requires a positive exponent which entails
diverging trajectories. Here we show that this is not enough. An additional
necessary condition for the emergence of chaos in the region where the
trajectory of the system goes through, is that the product of the maximal
positive exponent times the duration in which the system configuration point
stays in the unstable region should exceed unity. We give a theoretical
analysis justifying this result and a few examples.
