KononovychT. O. “Lower Bound for the Best Approximations of Periodic Summable Functions of Two Variables and Their Conjugates in Terms of Fourier Coefficients”. Ukrains’kyi Matematychnyi Zhurnal 60, no. 8 (August 25, 2008): 1042–1050. Accessed November 23, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/3222.