1. Browder, F., Nonlinear operators and nonlinear equations in Banach spaces, Nonlinear Functional Analysis, Proc. Symp. Pure Math., Vol. 18, Part 2.
2. Well set variational problems for multivalued operator equations;Dinca,1987
3. A variational method for nonlinear operators;Ionescu,1982
4. A variational proof of Friedrichs theorem;Ionescu;Anal. Univ. Bucharest,1984