1. F. I. Frankl, Selected Works on Gas Dynamics (Nauka, Moscow, 1973) [in Russian].
2. I. N. Vekua, Generalized Analytic Functions (GIFML, Moscow, 1959) [in Russian].
3. A. V. Bitsadze, ‘‘On a three-dimensional analogue of the Tricomi problem,’’ Sib. Math. J. 3, 642–644 (1962).
4. A. M. Ezhov and S. P. Pul’kin, ‘‘An estimate for the solution of the Tricomi problem for a class of equation of mixed type,’’ Dokl. Akad. Nauk SSSR 193, 978–980 (1970).
5. A. M. Nakhushev, ‘‘A three-dimensional analog of Gellerstedt’s problem,’’ Differ. Uravn. 4, 52–62 (1968).