Er’omenkoV. O. “Periodic Solutions of Systems of Two Linear First-Order Ordinary Differential Equations With Degenerate Asymmetric Matrix With Derivatives”. Ukrains’kyi Matematychnyi Zhurnal 50, no. 3 (March 25, 1998): 350–356. Accessed July 27, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/4936.