1. V. I. Smirnov, Course in Higher Mathematics [in Russian], Vol. 5, Fizmatgiz, Moscow (1959).
2. V. S. Vladimirov, Equations of Mathematical Physics [in Russian], Izd. Nauka, Moscow (1967).
3. G. N. Polozhii Equations of Mathematical Physics [in Russian], Izd. Vysshaya Shkola, Moscow (1964).
4. B. T. Polyak, ?Existence and convergence theorems for minimizing sequences in extremum problems subject to constraints,? Dokl. Akad. Nauk SSSR,166, No. 2 (1966).
5. J. L. Lions, Controle Optimal de Systemes Gouvernes par des Equations aux Derivees Partielles, Gauthier Villars, Dumod, Paris (1968).