Pelyukh, G. P. “On the Existence and Properties of Periodic Solutions of Discrete Difference Equations”. Ukrains’kyi Matematychnyi Zhurnal 46, no. 10 (October 25, 1994): 1382–1387. Accessed June 2, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/5617.