V. D. Nguyen, V. D. Nguyen, and K. C. Tran, “Van der Pol oscillator under parametric and forced excitations”, Ukr. Mat. Zhurn., vol. 59, no. 2, pp. 206–216, Feb. 2007, Accessed: Dec. 06, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/3304