Velikoivanenko, I. M., Lyashko, I. I., & Mistetsky, G. E. (1971). Solution in finite-difference stating of boundary value problems for one equation of elliptical type. Ukrains’kyi Matematychnyi Zhurnal, 23(2), 229-235. https://umj.imath.kiev.ua/index.php/umj/article/view/8505