V. O. Er’omenko, “On periodic solutions of linear degenerate second-order ordinary differential equations”, Ukr. Mat. Zhurn., vol. 49, no. 8, pp. 1137–1142, Aug. 1997, Accessed: Apr. 07, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/5107