FIRMAN, T.I. Countable hyperbolic systems in the theory of nonlinear oscillations. Carpathian Mathematical Publications, [S. l.], v. 8, n. 1, p. 163–171, 2016. DOI: 10.15330/cmp.8.1.163-171. Disponível em: https://scijournals.pnu.edu.ua/index.php/cmp/article/view/1422. Acesso em: 2 may. 2024.