IGNAT'EV, A. O. Bounds for the Periods of Periodic Solutions of Ordinary Differential Equations. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 67, n. 11, p. 1569–1573, 2015. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/2090. Acesso em: 5 dec. 2025.