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2. F. I. Frankl, ‘‘On Chaplygin problems for mixed subsonic and supersonic flows,’’ Izv. Akad. Nauk, Ser. Mat. 9, 121–142 (1945).
http://mi.mathnet.ru/im3631.
3. I. N. Vekua, Generalized Analytic Functions (Fizmatgiz, Moscow, 1959) [in Russian].
4. A. V. Bitsadze, Boundary Value Problems for Second Order Elliptic Equations (Nauka, Moscow, 1966) [in Russian].
5. A. V. Bitsadze, Some Classes of Partial Differential Equations (Nauka, Moscow, 1981) [in Russian].