1. V. BARBU, The time optimal control problem for parabolic variational inequalities, Applied Mathematics and Optimiz., 11: 1–22 (1984).
2. V. BARBU, Optimal Control of Variational Inequalities, Pitman Research Notes in Mathematics 100 London.Boston.Melbourne, 1984.
3. Lecture Notes in Mathematics;V. Barbu,1985
4. H. BREZIS, Problèmes unilatéraux. J. Math. Pures Appl., 51, 1–168 (1972).
5. H.O. FATTORINI, The maximum principle for nonlinear nonconvex systems in infinite dimensional spaces, Conference on Control Theory for Distributed Parameter Systems, Lecture Notes in Mathematics, Springer Verlag (to appear).