Pichugov, S. A. 2012. “Lower Bounds for the Deviations of the Best Linear Methods of Approximation of Continuous Functions by Trigonometric Polynomials”. Ukrains’kyi Matematychnyi Zhurnal 64 (5), 662-73. https://umj.imath.kiev.ua/index.php/umj/article/view/2606.