1. A. Zygmund, Trigonometric Series, 2nd ed. (Cambridge Univ. Press, Cambridge, 1959; Mir, Moscow, 1965), Vols. 1, 2.
2. S. V. Bochkarev, “Fourier Coefficients of Functions of Class Lip α with Respect to Complete Orthonormalized Systems,” Mat. Zametki 7(4), 397–402 (1970) [Math. Notes 7, 239–242 (1970)].
3. S. V. Bochkarev, “Absolute Convergence of Fourier Series with Respect to Complete Orthonormal Systems,” Usp. Mat. Nauk 27(2), 53–76 (1972) [Russ. Math. Surv. 27, 55–81 (1972)].
4. A. Zygmund, “Remarque sur la convergence absolue des séries de Fourier,” J. London Math. Soc. 3, 194–196 (1928).
5. R. Salem, “On a Theorem of Zygmund,” Duke Math. J. 10, 23–31 (1943).