MIHAILOVA, A.; GEORGIEV, N. An estimate of the period of periodical solution of an autonomous system of differential equations. Ann. Sofia Univ. Fac. Math. and Inf., [S. l.], v. 91, p. 107–114, 1999. Disponível em: https://stipendii.uni-sofia.bg/index.php/fmi/article/view/273. Acesso em: 4 dec. 2024.