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2. P. E. Sobolevskii, On Difference Methods for the Approximate Solution of Differential Equations (Voronezh State University Press, Voronezh, Russia, 1975).
3. M. G. Karatopraklieva, Differensial’nye Uravneniya 27, 68 1991 (in Russian).
4. D. Bazarov and H. Soltanov, Some Local and Nonlocal Boundary Value Problems for Equations of Mixed and Mixed-Composite Types (Ylym, Ashgabat, 1995, in Russian).
5. S. N. Glazatov, Nonlocal Boundary Value Problems for Linear and Nonlinear Equations of Variable Type (Sobolev Institute of Mathematics SB RAS, Novosibirsk, 1998, in Russian).