1. S. G. Mikhlin, Linear Partial Differential Equations (Vysshaya Shkola, Moscow, 1977) [in Russian].
2. V. P. Mikhailov, Partial Differential Equations (Nauka, Moscow, 1976) [in Russian].
3. L. Bers, F. John, and M. Schechter, Partial Differential Equations (Am. Math. Soc., Providence, RI, 1964).
4. D. Gilbarg and N. S. Trudinger, Elliptic Partial Differential Equations of Second Order (Springer, Berlin, 1983).
5. L. C. Evans, Partial Differential Equations (Am. Math. Soc., Providence, RI, 1998; 2010).