Pelyukh, G. P. “On Periodic Solutions of Nonlinear Difference Equations in the Critical Case”. Ukrains’kyi Matematychnyi Zhurnal 50, no. 2 (February 25, 1998): 304–308. Accessed April 16, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/4964.