Lopatin, A. K., and Yu. A. Mitropolskiy. “Bogolyubov Averaging and Normalization Procedures in Nonlinear Mechanics. I”. Ukrains’kyi Matematychnyi Zhurnal, vol. 46, no. 9, Sept. 1994, pp. 1171–1188, https://umj.imath.kiev.ua/index.php/umj/article/view/5634.