Martynyuk, D. I., N. A. Perestyuk, and A. M. Samoilenko. 1994. “Reducibility of Nonlinear Almost Periodic Systems of Difference Equations Given on a Torus”. Ukrains’kyi Matematychnyi Zhurnal 46 (4): 404–410. https://umj.imath.kiev.ua/index.php/umj/article/view/5737.