LOPATIN, A. K.; MITROPOLSKIY, Yu. A. Bogolyubov averaging and normalization procedures in nonlinear mechanics. IV. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 47, n. 8, p. 1044–1068, 1995. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5502. Acesso em: 5 dec. 2025.