LopatinA. K., and MitropolskiyYu. A. “Bogolyubov Averaging and Normalization Procedures in Nonlinear Mechanics. II”. Ukrains’kyi Matematychnyi Zhurnal 46, no. 11 (November 25, 1994): 1509–1526. Accessed November 24, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/5590.