Danylyuk, I. M., R. I. Petryshyn, and A. M. Samoilenko. “Averaging of Initial-Value and Multipoint Problems for Oscillation Systems With Slowly Varying Frequencies and Deviated Argument”. Ukrains’kyi Matematychnyi Zhurnal 59, no. 3 (March 25, 2007): 412–430. Accessed April 24, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/3316.