A. K. Lopatin and Y. A. Mitropolskiy, “Bogolyubov averaging and normalization procedures in nonlinear mechanics. IV”, Ukr. Mat. Zhurn., vol. 47, no. 8, pp. 1044–1068, Aug. 1995, Accessed: Dec. 05, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/5502