Serdyuk, A. S., and T. A. Stepanyuk. “Order Estimates for the Best Approximations and Approximations by Fourier Sums in the Classes of Convolutions of Periodic Functions of Low Smoothness in the Uniform Metric”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 66, no. 12, Dec. 2014, pp. 1658–1675, https://umj.imath.kiev.ua/index.php/umj/article/view/2253.