1. R. Courant and G. Robbins, What is Mathematics? An Elementary Approach to Ideas and Methods (Oxford Univ. Press, New York, 1941; RKhD, Moscow-Izhevsk, 2001).
2. A. O. Ivanov and A. A. Tuzhilin, Theory of Extremal Nets, in Contemporary Mathematics (IKI, Moscow-Izhevsk, 2003) [in Russian].
3. I. Ekeland and R. Temam, Convex Analysis and Variational Problems (North-Holland, Amsterdam, 1976; Mir, Moscow, 1979).
4. N. Dunford and J. Schwartz, Linear Operators, Vol. 1: General Theory (Interscience, New York-London, 1958; URSS, Moscow, 2004) [in Russian].