KononovychT. O. “Estimate for the Best Approximation of Summable Functions of Several Variables With a Certain Symmetry of Fourier Coefficients”. Ukrains’kyi Matematychnyi Zhurnal 55, no. 8 (August 25, 2003): 1138-1142. Accessed November 22, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/3987.