Serdyuk, A. S., and T. A. Stepanyuk. “Order Estimates for the Best Orthogonal Trigonometric Approximations of the Classes of Convolutions of Periodic Functions of Low Smoothness”. Ukrains’kyi Matematychnyi Zhurnal 67, no. 7 (July 25, 2015): 916–936. Accessed October 2, 2022. http://umj.imath.kiev.ua/index.php/umj/article/view/2033.