1. N. K. Bary, On best approximation of two conjugate functions by trigonometric polynomials, Izv. Akad. Nauk SSSR, 19 (1955), 285–302 (in Russian).
2. N. K. Bary and S. B. Stechkin, Best approximations and differential properties of two conjugate functions, Trudy Moskov. Mat. Obshch., 5 (1956), 483–522 (in Russian).
3. A. Belov, M. Dyachenko and S. Tikhonov, Functions with general monotone Fourier coefficients, Russian Math. Surveys, 76 (2021), 951-1017
4. translated from Uspekhi Mat. Nauk, 76:6 (2021), 3-70.
5. R. P. Boas, Fourier series with positive coefficients, J. Math. Anal. Appl., 17 (1967), 463–483.