ZadereiP. V., and Smal’. B. M. “Estimates for the Best Approximation and Integral Modulus of Continuity of a Function in Terms of Its Fourier Coefficients”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 50, no. 4, Apr. 1998, pp. 496–503, https://umj.imath.kiev.ua/index.php/umj/article/view/4919.