Bushev, D. M., and Y. I. Kharkevych. “Approximation of Classes of Periodic Multivariable Functions by Linear Positive Operators”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 58, no. 1, Jan. 2006, pp. 12–19, https://umj.imath.kiev.ua/index.php/umj/article/view/3430.