Lopatin, A. K., & Mitropolskiy, Y. A. (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