1. I. V. Skrypnik, Nonlinear Elliptic Equations of Higher Order [in Russian], Naukova Dumka, Kiev (1973).
2. L. S. Pontryagin, Foundations of Combinatorial Topology [in Russian], Nauka, Moscow (1976).
3. F. E. Browder, "Pseudomonotone operators and the direct method of variations," Arch. Rational Mech. Anal.,38, No. 4, 268?277 (1970).
4. J. Milnor and A. Wallace, Differential Topology. First Course [Russian translation], Mir, Moscow (1972).
5. M. A. Krasnosel'skii and P. P. Zabreiko, Geometric Methods of Nonlinear Analysis [in Russian], Nauka, Moscow (1975).