SerdyukA. S. “Approximation of Periodic Analytic Functions by Interpolation Trigonometric Polynomials in the Metric of the Space $L$”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 54, no. 5, May 2002, pp. 692-9, https://umj.imath.kiev.ua/index.php/umj/article/view/4108.