KOROL', I. I. Investigation of the periodic solutions of nonlinear autonomous systems in the critical case. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 60, n. 3, p. 332–339, 2008. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/3159. Acesso em: 6 dec. 2025.