Time correlation functions and spectral distributions of the duffing oscillator in a random force field

The Brownian motion of the Duffing oscillator is analyzed in the case when the oscillator damping is small compared with its frequency, whereas the nonlinearity may be arbitrary. The expressions for the time-correlation functions of coordinates are obtained in an explicit form. If the nonlinearity is small, the dynamics of the system is shown to be determined by a relation between the frequency straggling due to fluctuations of the amplitude and damping. At large nonlinearity the correlators do not depend on the damping. The frequency dependences of the spectral representations of the correlators of coordinates are investigated for various ratios between the oscillator parameters.