Dinh, Nguyen Van, and Tran Duong Tri. “The Poincaré Method for an Oscillator With Quadratic Nonlinearity”. Vietnam Journal of Mechanics 26, no. 1 (April 1, 2004): 23–30. Accessed December 6, 2025. https://vjs.ac.vn/vjmech/article/view/5686.