“…These methods include the harmonic balance method [16,17], variational iteration method [18,19], homotopy analysis method [20,21], homotopy perturbation method [22,23], Li-He's modified homotopy perturbation method [24], enhanced homotopy perturbation method [25], asymptotic method [26,27], energy balance method [28], differential transformation method [29], parameter expansion method [30,31], variational principle [32], and frequency-amplitude formulation [33], among others. In this paper, the nonlinear oscillation with cubic and harmonic restoring force [1,34,35] will be explored:…”