Er’omenkoV. O. (1998) “Periodic solutions of systems of two linear first-order ordinary differential equations with degenerate asymmetric matrix with derivatives”, Ukrains’kyi Matematychnyi Zhurnal, 50(3), pp. 350–356. Available at: https://umj.imath.kiev.ua/index.php/umj/article/view/4936 (Accessed: 24November2024).