1. N. P. Korneichuk, Extremal Problems of Approximation Theory (Nauka, Moscow, 1976) [in Russian].
2. N. P. Korneichuk, A. A. Ligun, and V. G. Doronin, Approximation with Constraints (Naukova Dumka, Kiev, 1982) [in Russian].
3. M. A. Krasnosel’skii and Y. B. Rutitskii, Convex Functions and Orlicz Spaces, Problems of Contemporary Mathematics, (Fizmatgiz, Moscow, 1958) [in Russian].
4. S. G. Krein, Yu. I. Petunin, and E. M. Semenov, Interpolation of Linear Operators (Nauka, Moscow, 1978) [in Russian].
5. H. Triebel, Interpolation Theory, Function Spaces, Differential Operators (Birkhäuser, Berlin, 1977; Mir, Moscow, 1980).