LEONOV, G. A. On the problem of estimation of the number of cycles in two-dimensional quadratic systems from the viewpoint of nonlinear mechanics. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 50, n. 1, p. 48–57, 1998. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/4971. Acesso em: 5 dec. 2025.