Y. I. Bigun, “Existence of a solution and averaging of nonlinear multifrequency problems with delay”, Ukr. Mat. Zhurn., vol. 59, no. 4, pp. 435–446, Apr. 2007, Accessed: Dec. 05, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/3318