Y. I. Bigun, “Averaging of Oscillation Systems with Delay and Integral Boundary Conditions”, Ukr. Mat. Zhurn., vol. 56, no. 2, pp. 257–263, Feb. 2004, Accessed: Apr. 13, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/3749