LOPATIN, A. K.; MITROPOLSKIY, Yu. A. Bogolyubov averaging and normalization procedures in nonlinear mechanics. I. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 46, n. 9, p. 1171–1188, 1994. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5634. Acesso em: 11 apr. 2025.