TEREKHIN, M. T. Existence of Small Periodic Solutions of Nonlinear Systems of Ordinary Differential Equations. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 53, n. 5, p. 680–687, 2001. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/4289. Acesso em: 14 apr. 2025.