Lykova О. B.; BOICHUK, A. A. Construction of periodic solutions of non-linear systems in critical cases . Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 40, n. 1, p. 62–69, 1987. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/9056. Acesso em: 6 dec. 2025.