Bushev, D. M., and Yu. I. Kharkevych. “Approximation of Classes of Periodic Multivariable Functions by Linear Positive Operators”. Ukrains’kyi Matematychnyi Zhurnal 58, no. 1 (January 25, 2006): 12–19. Accessed May 2, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/3430.