First, we recall briefly some known results of deterministic motions in (1) for the case of smallness of the coefficients. It is supposed that is close to the natural frequency , namely :
, (2)
here is a detuning parameter and is a small positive one .Applying to (1) the asymptotic method [2] and using the amplitude and phase variables () given by
(3)
is close 
, namely :
, (2)
is a detuning parameter and 
is a small positive one .Applying to (1) the asymptotic method [2] and using the amplitude and phase variables (
) given by
(3)