1. A. Friedman, Variational Principles and Free Boundary Problems (Wiley-Interscience, New York, 1982; Mir,Moscow, 1990).
2. F. Donati, “A Penalty Method Approach to Strong Solutions of Some Nonlinear Parabolic Unilateral Problems,” Nonlinear Anal. 6, 585–597 (1982).
3. R. Glowinski, J.-L. Lions, and R. Trémolières, Numerical Analysis of Variational Inequalities (North-Holland Publishing Company, Amsterdam, 1981; Mir, Moscow, 1979).
4. M. Boman, A Posteriori Error Analysis in the Maximum Norm for a Penalty Finite Element Method for the Time-Dependent Obstacle Problem (Chalmers University of Technology and Goteborg University, Amsterdam, 2000).
5. R. Scholz, “Numerical Solution of the Obstacle Problem by the Penalty Method,” II. Time-Dependent Problems Numer. Math. 49, 255–268 (1986).