Yusupov, Gulzorkhon Amirshoevich. “Best Polynomial Approximations and Widths of Certain Classes of Functions in the Space \( L_2 \)”. Eurasian Mathematical Journal 4, no. 3 (September 30, 2013): 120–126. Accessed February 23, 2026. https://emj.enu.kz/index.php/main/article/view/559.