SerdyukA. S. (2000). Approximation of Periodic Functions of High Smoothness by Interpolation Trigonometric Polynomials in the Metric of $L_1$. Ukrains’kyi Matematychnyi Zhurnal, 52(7), 994-998. Retrieved from https://umj.imath.kiev.ua/index.php/umj/article/view/4501