Gorodnii, M. F. 1995. “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 47 (4): 535–541. https://umj.imath.kiev.ua/index.php/umj/article/view/5453.