Pelyukh, G. P. “On the Existence of Periodic Solutions of Nonlinear Difference Equations”. Ukrains’kyi Matematychnyi Zhurnal 54, no. 12 (December 25, 2002): 1626–1633. Accessed April 7, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/4201.