Samoilenko, A. M., and Yu. V. Teplinsky. 1998. “Invariant Tori of Linear Countable Systems of Discrete Equations Given on an Infinite-Dimensional Torus”. Ukrains’kyi Matematychnyi Zhurnal 50 (2): 244–251. https://umj.imath.kiev.ua/index.php/umj/article/view/4958.