Aldashev, S. A. 1997. “Many-Dimensional Dirichlet and Tricomi Problems for One Class of Hyperbolic-Elliptic Equations”.
Ukrains’kyi Matematychnyi Zhurnal
49 (12): 1587–1593.
https://umj.imath.kiev.ua/index.php/umj/article/view/5162
.