Aldashev, S. A. “Many-Dimensional Dirichlet and Tricomi Problems for One Class of Hyperbolic-Elliptic Equations”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 49, no. 12, Dec. 1997, pp. 1587–1593, https://umj.imath.kiev.ua/index.php/umj/article/view/5162.