I. M. Danylyuk, R. I. Petryshyn, and A. M. Samoilenko, “Averaging of initial-value and multipoint problems for oscillation systems with slowly varying frequencies and deviated argument”, Ukr. Mat. Zhurn., vol. 59, no. 3, pp. 412–430, Mar. 2007, Accessed: Dec. 05, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/3316