A. M. Samoilenko and R. I. . Petrishin, “Averaging method in multifrequency systems with slowly varying parameters”, Ukr. Mat. Zhurn., vol. 40, no. 4, pp. 493–500, Jun. 1988, Accessed: Dec. 06, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/9185