1. Bers, L.: Mathematical aspects of subsonic and transonic gas dynamics, Wiley, New York, 1958
2. Bitsadze, A. V.: Some classes of partial differential equations, Gordon and Breach, New York, 1988
3. Rassias, J. M.: Lecture notes on mixed type partial differential equations, World Scientific, Singapore, 1990
4. Smirnov, M. M.: Equations of mixed type, Amer. Math. Soc., Providence RI, 1978
5. Sun H. S.: Tricomi problem for nonlinear equation of mixed type. Science in China, Series A, 35, 14–20 (1992)