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2. R. Courant, Partielle Differentialgleichungen, unpublished lecture notes (Göttingen, 1932; Mir, Moscow, 1964).
3. F. John, Plane Waves and Spherical Means Applied to Partial Differential Equations (Interscience, New York, 1955; Inostrannaya Literatura, Moscow, 1958).
4. Gel’fand, I.M., Graev, M.I., and Vilenkin, N.Ya., Integral Geometry and Related Questions of Representation Theory (Fizmatgiz, Moscow, 1962) [in Russian].
5. M. M. Lavrent’ev and L. Ya. Savel’ev, Operator Theory and Ill-Posed Problems (Inst. Mat., Sib. Otd. Ross. Akad. Nauk, Novosibirsk, 1999) [in Russian].