1. Baiocchi, C. and Capello, A., Variational and Quasivariational Inequalities. Applications to Free Boundary Problems, New York: Wiley, 1984. Translated under the title Variatsionnye i kvazivariatsionnye neravenstva. Prilozheniya k zadacham so svobodnoi granitsei, Moscow: Nauka, 1988.
2. Noor, M.A., J. Math. Anal. Appl., 1988, vol. 130, no. 2, pp. 344–353.
3. Siddiqi, A.H. and Ansari, Q.H., J. Math. Anal. Appl., 1990, vol. 149, no. 2, pp. 444–450.
4. Siddiqi, A.H. and Ansari, Q.H., J. Math. Anal. Appl., 1992, vol. 166, no. 2, pp. 386–392.
5. Ryazantseva, I.P., Zh. Vychisl. Mat. Mat. Fiz., 2007, vol. 47, no. 2, pp. 186–193.