TEREKHIN, M. T. Nontrivial periodic solutions of a nonautonomous system of second-order differential equations. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 46, n. 6, p. 754–759, 1994. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5703. Acesso em: 11 apr. 2025.