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3. Sh. Chen, ‘‘Mixed type equations in gaz dynamics,’’ Quart. Appl. Math. 68, 487–511 (2010).
4. F. Natterer, The Mathematics of Computerized Tomography (SIAM, Philadelphia, 2001).
5. M. M. Smirnov, Degenerating Elliptic and Hyperbolic Equations (Visheysh. Shkola, Minsk, 1977) [in Russian].