A new method based on an incremental energy approach and the standard
mapping technique is proposed for the study of resonant layers in nonl
inear dynamics. To demonstrate the procedure, the method is applied to
four types of Duffing oscillators. The appearance, disappearance and
accumulated disappearance strengths of the resonant layers for each ty
pe of oscillator are derived. A quantitative check of the appearance s
trength is performed by computing its value using three independent me
thods: Chirikov overlap criterion, renormalization group technique, an
d numerical simulations. It is also observed that for the case of the
twin-well Duffing oscillator, its perturbed left and right wells are a
symmetric.