Konopovich TO. Estimation of the best approximation of periodic functions of two variables by an “angle” in the metric of $L_p$. Ukr. Mat. Zhurn. [Internet]. 2004Sep.25 [cited 2024Nov.24];56(9):1182–1192. Available from: https://umj.imath.kiev.ua/index.php/umj/article/view/3834