Y. A. Mitropolskiy and G. P. Khoma, “On the periodic solutions of the second-order wave equations. V”, Ukr. Mat. Zhurn., vol. 45, no. 8, pp. 1115–1121, Aug. 1993, Accessed: Dec. 05, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/5908