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, [S. l.], v. 50, n. 3, p. 350–356, 1998. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/4936. Acesso em: 5 dec. 2025.