1. W. Rudin, Principles of Mathematical Analysis, 3rd ed. (McGraw-Hill, New York, 1976; Mir, Moscow, 1976).
2. W. Rudin, Functional Analysis (McGraw-Hill, New York, 1973; Mir, Moscow, 1975).
3. Yu. A. Kaz’min, “Closure of the linear hull of a system of functions,” Sibirsk. Mat. Zh. 18(4), 799–805 (1977) [Siberian Math. J. 18 (4), 565–570 (1977)].