Er’omenko, V. O. “Periodic Solutions of Systems of Two Linear First-Order Ordinary Differential Equations With Degenerate Asymmetric Matrix With Derivatives”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 50, no. 3, Mar. 1998, pp. 350–356, https://umj.imath.kiev.ua/index.php/umj/article/view/4936.