Bigun, Ya. I. “Existence of a Solution and Averaging of Nonlinear Multifrequency Problems With Delay”. Ukrains’kyi Matematychnyi Zhurnal 59, no. 4 (April 25, 2007): 435–446. Accessed December 6, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/3318.