RONTO, A. M. On the Periods of Periodic Motions in Autonomous Systems. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 53, n. 1, p. 94–112, 2001. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/4225. Acesso em: 7 apr. 2025.