In a previous paper, the authors deduced a LEM representation for the damped Düffing oscillator with harmonic forcing, in null Cauchy conditions; the undamped case could not be obtained from this by canceling the damping coefficient. Therefore, in this paper they consider directly the harmonically forced undamped Düffing oscillator with nonzero initial conditions, establishing for it analytic LEM approximate solutions tested by using classical numerical methods. Some representative phase portraits are also presented.

Keywords: Düffing oscillator, linear equivalence method

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