ER’OMENKO, V. O. On periodic solutions of linear degenerate second-order ordinary differential equations. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 49, n. 8, p. 1137–1142, 1997. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5107. Acesso em: 7 apr. 2025.