DanilovV. Ya., MartynyukD. I., and Pan’kovV. G. “Second Bogolyubov Theorem for Systems of Difference Equations”. Ukrains’kyi Matematychnyi Zhurnal 48, no. 4 (April 25, 1996): 464-475. Accessed November 24, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/5311.