Bigun, Ya. I. “Averaging of Oscillation Systems With Delay and Integral Boundary Conditions”. Ukrains’kyi Matematychnyi Zhurnal 56, no. 2 (February 25, 2004): 257-263. Accessed May 5, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/3749.