Gorodnii, M. F. “Approximation of a Bounded Solution of a Linear Difference Equation on Z2 by Solutions of Corresponding Boundary-Value Problems in a Banach Space”. Ukrains’kyi Matematychnyi Zhurnal, vol. 47, no. 4, Apr. 1995, pp. 535–541, https://umj.imath.kiev.ua/index.php/umj/article/view/5453.