V. O. Er’omenko, “Periodic solutions of systems of two linear first-order ordinary differential equations with degenerate asymmetric matrix with derivatives”, Ukr. Mat. Zhurn., vol. 50, no. 3, pp. 350–356, Mar. 1998, Accessed: Feb. 17, 2026. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/4936