RONTO , N. I.; MARTYNYUK , O. M. Study of periodic solutions of the second-order notations. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 44, n. 1, p. 83–93, 1992. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/7798. Acesso em: 6 dec. 2025.