B. T. Nohara, “Nonexistence theorem except the out-of-phase and in-phase solutions in the coupled van der Pol equation system”, Ukr. Mat. Zhurn., vol. 61, no. 8, pp. 1106–1129, Aug. 2009, Accessed: Dec. 06, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/3084