1. O. A. Ladyzhenskaya and N. N. Uraltseva, Linear and Quasilinear Equations of Elliptic Type [in Russian], Nauka, Moscow (1964);
2. English transl.: Academic Press, New York etc. (1968).
3. D. Gilbarg and N. S. Trudinger, Elliptic Partial Differential Equations of Second Order, Springer, Berlin (2001).
4. D. Kinderlehrer and G. Stampacchia, An Introduction to Variational Inequalities and Their Applications Academic Press, New York etc. (1980).
5. B. V. Bojarski, “Generalized solutions of a system of first order differential equations of elliptic type with discontinuous coefficients” [in Russian], Mat. Sb. 43, No. 4, 451–503 (1957).