1. B. S. Kashin and B. S. Saakyan, Orthogonal Series (AFC Publ., Moscow, 1999) [in Russian].
2. S. Kachmarz and H. Steinhaus, Theorie der Orthogonalreihen (Chelsea Publishing Co., N.Y., 1951; Fizmatgiz, Moscow, 1958).
3. G. Aleksich, Problems of Convergence of Orthogonal Series (Nauka, Moscow, 1984) [in Russian].
4. A. N. Kolmogorov and S. V. Fomin, Elements of the Theory of Functions and Functional Analysis (Nauka, Moscow, 1976; Dover Publ., 1999).
5. V. I. Bogachev and O. G. Smolyanov, Real and Functional Analysis. University Course (NITs RKhD, Moscow, Izhevsk, 2009) [in Russian].