N. A. Perestyuk and A. B. Tkach, “Periodic solutions of a weakly nonlinear system of partial differential equations with pulse influence”, Ukr. Mat. Zhurn., vol. 49, no. 4, pp. 601–605, Apr. 1997, Accessed: Dec. 05, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/5038