GREBENIKOV, E. A. On one mathematical problem in the theory of nonlinear oscillations. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 60, n. 1, p. 56–62, 2008. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/3136. Acesso em: 5 dec. 2025.