Lopatin, A. K., and Yu. A. Mitropolskiy. 1994. “Bogolyubov Averaging and Normalization Procedures in Nonlinear Mechanics. I”.
Ukrains’kyi Matematychnyi Zhurnal
46 (9): 1171–1188.
https://umj.imath.kiev.ua/index.php/umj/article/view/5634
.