N. A. Marchuk and Y. V. Teplinsky, “On the Smoothness of the Invariant Torus of a Countable System of Difference Equations with Parameters”, Ukr. Mat. Zhurn., vol. 53, no. 9, pp. 1241–1250, Sep. 2001, Accessed: Dec. 05, 2025. [Online]. Available: https://umj.imath.kiev.ua/index.php/umj/article/view/4343