1. A. S. Belov, “On the sums of moduli of terms of a grouped trigonometric series with monotonic coefficients,” Vestn. Ivanov. Gos. Univ., Ser. Biol. Khimiya Fiz. Mat., No. 3, 107–121 (2006).
2. A. S. Belov, “Some properties of the sum of the moduli of the terms of a grouped trigonometric series,” Sb. Math. 203 (6), 798–825 (2012).
3. A. S. Belov and S. A. Telyakovskii, “Refinement of the Dirichlet–Jordan and Young’s theorems on Fourier series of functions of bounded variation,” Sb. Math. 198 (6), 777–791 (2007).
4. V. P. Zastavnyi, “Estimates for sums of moduli of blocks in trigonometric Fourier series,” Proc. Steklov Inst. Math. 273, Suppl. 1, S190–S204 (2011).
5. A. Zygmund, Trigonometric Series (Cambridge Univ. Press, New York, 1959; Mir, Moscow, 1965), Vols. 1, 2.