Lopatin, A. K., and Yu. A. Mitropolskiy. “Bogolyubov Averaging and Normalization Procedures in Nonlinear Mechanics. I”. Ukrains’kyi Matematychnyi Zhurnal 46, no. 9 (September 25, 1994): 1171–1188. Accessed December 5, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/5634.