Samoilenko, A.M. (1998) “Asymptotic method for the investigation of m-frequency oscillation systems”,
Ukrains’kyi Matematychnyi Zhurnal, 50(10), pp. 1366–1387. Available at:
https://umj.imath.kiev.ua/index.php/umj/article/view/4823 (Accessed: 10 April 2025).