Petryshyn, Ya. R., and A. M. Samoilenko. “Averaging Method in Multipoint Problems of the Theory of Nonlinear Oscillations”. Ukrains’kyi Matematychnyi Zhurnal 48, no. 8 (August 25, 1996): 1096–1103. Accessed December 6, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/5253.