Pichugov, S. A. “Lower Bounds for the Deviations of the Best Linear Methods of Approximation of Continuous Functions by Trigonometric Polynomials”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 64, no. 5, May 2012, pp. 662-73, https://umj.imath.kiev.ua/index.php/umj/article/view/2606.