Fan, Jie. “Application of He’s Frequency-Amplitude Formulation to the Duffing-Harmonic Oscillator”. Topological Methods in Nonlinear Analysis 31, no. 2 (June 1, 2008): 389–394. Accessed March 29, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.2008.031.