1. Babič, V. M.: On the extension of functions, Uspekhi Mat. Nauk8 (1953), 111–113 (Russian).
2. 1. Besov, O. V., Il'in, V.P. and Nikol'ski??, S. M.: Integral representation of functions and embedding theorems, 1-st ed.-"nauka", Moscow, 1975 (Russian)
3. 2. 2-nd ed.-"nauka", Moscow, 1996 (Russian)
4. 3. English transi, of 1-st ed., Vols, l, 2, Wiley, 1979.
5. Burago, Yu. D. and Maz’ya, V. G.: Some problems of the potential theory and function theory for domains with irregular boundaries, Zap. Nauchn. Semin. Leningr. Otd. Mat. Inst. Steklov 3 (1967), 1–152 (Russian); English transl, in Seminars in Math., Steklov Math. Inst.,Leningrad} 3 (1969).