Lopatin, A. K., and Yu. A. Mitropolskiy. “Bogolyubov Averaging and Normalization Procedures in Nonlinear Mechanics. III”.
Ukrains’kyi Matematychnyi Zhurnal, vol. 46, no. 12, Dec. 1994, pp. 1627–1646,
https://umj.imath.kiev.ua/index.php/umj/article/view/5574.