1. Bers, L. Mathematical Aspects of Subsonic and Transonic Gas Dynamics (Wiley, New York, London, 1958).
2. Serbina, L. I. A Problem for the Linearized Boussinesq Equation With a Nonlocal Samarskii Condition, Diff. equat. 38, No. 8, 1187–1194 (2002).
3. Bitsadze, A. V. Some Classes of Partial Differential Equations (Nauka, Moscow, 1981) [in Russian].
4. Gilbert, R. Function Theoretic Methods in Partial Differential Equations (Academic Press, New York, 1969).
5. Smirnov, M. M. Degenerate Elliptic and Hyperbolic Equations (Nauka, Moscow, 1966) [in Russian].