“About $C^\varrho$-smootness of an invariant torus of a countable system of difference equations defined on an $m$-dimensional torus” (2002) Neliniini Kolyvannya, 5(2), pp. 251–265. Available at: https://nosc.imath.kiev.ua/index.php/nosc/article/view/218 (Accessed: 12 April 2026).