Shydlich, A. L. “Saturation of the Linear Methods of Summation of Fourier Series in the Spaces S Pφ”. Ukrains’kyi Matematychnyi Zhurnal, vol. 60, no. 6, June 2008, pp. 815 – 828, https://umj.imath.kiev.ua/index.php/umj/article/view/3198.