Lopatin, A.K. and Mitropolskiy, Y.A. (1995) “Bogolyubov averaging and normalization procedures in nonlinear mechanics. IV”, Ukrains’kyi Matematychnyi Zhurnal, 47(8), pp. 1044–1068. Available at: https://umj.imath.kiev.ua/index.php/umj/article/view/5502 (Accessed: 5 December 2025).