[1] Oleinik O A, Radkevic E V. Second order equations with nonnegative characteristic form. American Math
Soc, New York: Rhode Island and Plenum Press, 1973

[2] Zhao Junning. First boundary value problem for quasilinear equations of second order with nonnegative
characteristic form. Chinese Annals of Math, 1983, 4A(4): 475-486

[3] Lady?zhenskaja O A, Ural’ceva N N. Linear and Quasilinear elliptic equations. Moscow: Nauka, 1964;
(Engl). New York: Academic Press, 1968

[4] Tartar L C. Compensated compactness and applications to partial differential equation. In: Nonlinear
Analysis and Mechanics, Heriot-Watt Symposium, IV, 136-193. Research Notes in Mathematics Pitman,
1979

[5] Vol’pert A I. BV spaces and quasilinear equations. Math SB, 1967, 73(115): 255-302