1. S. Yano, J. Math. Soc. Japan 3(2), 296 (1951).
2. A. Zygmund, Trigonometric Series (Cambridge, 1960; Mir, Moscow, 1965), Vols. 1, 2.
3. S. V. Astashkin and K. V. Lykov, Sibirsk. Mat. Zh. 47(5), 974 (2006) [SiberianMath. J. 47 (5), 797 (2006)].
4. S. V. Astashkin and K. V. Lykov, Sibirsk. Mat. Zh. 50(2), 250 (2009) [SiberianMath. J. 50 (2), 199 (2009)].
5. G. E. Karadzhov and M. Milman, J. Approx. Theory 133(1), 38 (2005).