Aldashev , S. A. “On Some Boundary Value Problems for Linear Multidimensional Hyperbolic Equations of the Second Order”. Ukrains’kyi Matematychnyi Zhurnal, vol. 43, no. 3, Feb. 1991, pp. 415-20, https://umj.imath.kiev.ua/index.php/umj/article/view/9624.