KONOPOVICH, T. O. Estimation of the best approximation of periodic functions of two variables by an “angle” in the metric of $L_p$. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 56, n. 9, p. 1182–1192, 2004. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/3834. Acesso em: 5 dec. 2025.