Nonsmooth critical point theory on closed convex sets and nonlinear hemivariational inequalities

In this paper we develop a critical point theory for nonsmooth locally Lipschitz functionals defined on a closed, convex set extending this way the work of Struwe (Variational Methods, Springer, Berlin, 1990). Through a deformation result, we obtain minimax principles producing critical points. Then we use the theory to obtain positive and negative solutions of nonlinear and semilinear hemivariational inequalities. In this context we improve a result on positive solutions for semilinear elliptic problems due to Nirenberg (Variational methods in nonlinear problems, in: Topics in Calculus of Variations, Lecture Notes in Mathematics, vol. 1365, Springer, Berlin, 1987).