Konopovich, T. O. (2004) “Estimation of the best approximation of periodic functions of two variables by an ‘angle’ in the metric of $L_p$”, Ukrains’kyi Matematychnyi Zhurnal, 56(9), pp. 1182–1192. Available at: https://umj.imath.kiev.ua/index.php/umj/article/view/3834 (Accessed: 14June2024).