1. S. M. Nikol'skii, Approximation of Functions of Several Variables and Inclusion Theorems [in Russian], Nauka, Moscow (1977).
2. M. I. Ginzburg, in: Studies on the Theory of Functions of Several Real Variables [in Russian], Yaroslavl? (1976), pp. 22?34.
3. V. A. Rodin, in: Proceedings of the Seventh Winter School, Drogobych, 1974. Theory of Functions and Functional Analysis [in Russian], Moscow (1976), pp. 133?140.
4. S. G. Krein and Yu. I. Petunin, Interpolation of Linear Operators [in Russian], Nauka, Moscow (1978).
5. L. V. Kantorovich and G. P. Akilov, Functional Analysis [in Russian], Nauka, Moscow (1978).