GorodniiM. F., and RomanenkoV. N. 2000. “Approximation of a Bounded Solution of One Difference Equation With Unbounded Operator Coefficient by Solutions of the Corresponding Boundary-Value Problems”. Ukrains’kyi Matematychnyi Zhurnal 52 (4), 548-52. https://umj.imath.kiev.ua/index.php/umj/article/view/4445.