PERESTYUK, N. A.; FEKETA, P. V. On Preservation of the Invariant torus for Multifrequency Systems. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 65, n. 11, p. 1498–1505, 2013. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/2529. Acesso em: 5 dec. 2025.