G. P. Pelyukh, “On the Existence of Periodic Solutions of Nonlinear Difference Equations”, Ukr. Mat. Zhurn., vol. 54, no. 12, pp. 1626–1633, Dec. 2002, Accessed: Apr. 07, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/4201