LAKUSTA, L. M.; PETRYSHYN, R. I. On integral manifolds of oscillating systems with slowly varying frequencies. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 50, n. 1, p. 87–93, 1998. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/4974. Acesso em: 14 apr. 2025.