Petryshyn, Y.R. and Samoilenko, A.M. (1996) “Averaging method in multipoint problems of the theory of nonlinear oscillations”, Ukrains’kyi Matematychnyi Zhurnal, 48(8), pp. 1096–1103. Available at: https://umj.imath.kiev.ua/index.php/umj/article/view/5253 (Accessed: 6 December 2025).