ВarkovskyV. V., and RoitbergY. A. 1966. “On the Minimum and Maximum Operator Corresponding to the General Elliptical Problem With Inhomogeneous Boundary Value Conditions ”. Ukrains’kyi Matematychnyi Zhurnal 18 (2), 91-97. https://umj.imath.kiev.ua/index.php/umj/article/view/8719.