1. L. Hörmander, Analysis of Partial Differential Operators (Springer, Berlin, 1983), Vols. I–IV.
2. G. Eskin, Boundary Value Problems for Elliptic Pseudodifferential Equations (AMS, Providence, RI, 1981).
3. A. Samarskii, The Theory of Difference Schemes (CRC, Boca Raton, FL, 2001).
4. V. Ryaben’kii, Method of Difference Potentials and its Applications (Springer, Berlin, 2002).
5. F. D. Gakhov, Boundary Value Problems (Dover, Mineola, NY, 1981).