TsvetkovE. L. “Smoothness of Generalized Solutions of the Third Boundary-Value Problem for an Elliptic Differential-Difference Equation”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 45, no. 8, Aug. 1993, pp. 1140–1150, https://umj.imath.kiev.ua/index.php/umj/article/view/5911.