1. V. N. Vragov, “On the theory of boundary value problems for equations of mixed type in space,” Differ. Equations 13 (6), 759–764(1977).
2. F. I. Frankl, Selected Works in Gas Dynamics (Nauka, Moscow, 1973) [in Russian].
3. M. M. Smirnov, Mixed Type Equations (Vyssh. Shk., Moscow, 1985) [in Russian].
4. M. S. Salakhitdinov and A. K. Urinov, On the Spectral Theory of Equations of Mixed Type (FAN, Tashkent, 1997) [in Russian].
5. T. D. Dzhuraev, Boundary Value Problems for Equations of Mixed and Mixed Composite Types (FAN, Tashkent, 1979) [in Russian].