1. Krasnosel’skii, M. A. and Zabreiko, P. P. Geometrical Methods of Nonlinear Analysis, (Nauka, Moscow, 1975; Springer-Verlag, Berlin-Heidelberg-New York-Tokyo, 1984).
2. Pokhozhaev, S. I. “Solvability of Nonlinear Equations with Odd Operators,” Funct. Anal. Appl. 1, No. 3, 227–233 (1967).
3. Skrypnik, I. V. Methods for Analysis of Nonlinear Elliptic Boundary-Value Problems, (Nauka, Moscow, 1990; AMS, 1994).
4. Bobylev, N. A., Emel’yanov, S. V., and Korovin, S. K. Geometrical Methods in Variational Problems (Magistr, Moscow, 1998) [in Russian].
5. Lions J. L. Quelques Méthodes de Résolution des Problèmes aux Limites non Linéaires, (Dunod; Gauthier-Villars, Paris, 1969; Mir, Moscow, 1972).