KononovychT. O. “Estimate for the Best Approximation of Summable Functions of Several Variables With a Certain Symmetry of Fourier Coefficients”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 55, no. 8, Aug. 2003, pp. 1138-42, https://umj.imath.kiev.ua/index.php/umj/article/view/3987.