  We study an autoresonant asymptotic behaviour for nonlinear oscillators under
slowly changing frequency and amplitude of external driver. As a result we
obtain formulas for threshold values of amplitude and frequency of the driver
when autoresonant behaviour for the nonlinear oscillator is observed. Also we
study a capture into resonance and emission out of the resonance for
trajectories of the oscillator. A measure of autoresonant asymptotic behaviours
for nonlinear oscillator is obtained.
