Tasmambetov , Zh. N. “One System of Differential Equations in Partial Derivatives of the Second Order”. Ukrains’kyi Matematychnyi Zhurnal 44, no. 3 (April 2, 1992): 427–430. Accessed April 14, 2025. https://umj.imath.kiev.ua/index.php/umj/article/view/8113.