1. Oleinik, O.A. and Radkevich, E.V., Second order equations with nonnegative characteristic form, AMS Rhode Island and Plenum Press, New York, 1973.
2. Shen, Y.T., On the Dirichlet problem for quasilinear elliptic equations with singular coefficients, Proc. 1980 Beijing international Symp. on DD, Vol. 3, pp, 1407–1417, sci. Press, 1982.
3. Browder, F. E.,Pseudo-monotone operator and nonlinear elliptic boundary value problem or unbounded domains, Proc. North. Acad. Sci. U.S.A.,74 (1977), 2659–2661.
4. Lions, J.L., Quelques mèthodes de rèsolution des problèmes aux limites non lineaires, Dunod and Gauthier-Villaro, Paris, 1969.