Dinh, Nguyen Van, and Tran Kim Chi. “A System of Van Der Pol Type under Nonlinear Parametric Excitation”. Vietnam Journal of Mechanics 19, no. 4 (December 30, 1997): 21–28. Accessed April 26, 2024. https://vjs.ac.vn/index.php/vjmech/article/view/10064.