1. Bensoussan, A., Lions, J.L.: Application of Variational Inequalities in Stochastic Control. Amsterdam, New York, Oxford: North Holland 1982
2. Berger, A.E., Falk, R.S.: An error estimate for the truncation method for the solution of parabolic obstacle variational inequalities. Math. Comput.31, 619?628 (1977)
3. Br�zis, H.: Probl�mes unilat�raux. J. Math. Pures Appl.51, 1?168 (1972)
4. Br�zis, H., Sibony, M.: Methodes d'approximation et d'it�ration pour les op�rateurs monotones. Archive Ration. Mech. Anal.28, 59?82 (1968)
5. Da Prato, G., Grisvard, P.: Somme d'op�rateurs lin�aires et �quations diff�rentielles op�rationelles. J. Math. Pures Appl.54, 305?387 (1975)