YUSUPOV, G. A. Best Polynomial Approximations and Widths of Certain Classes of Functions in the Space \( L_2 \). Eurasian Mathematical Journal, [S. l.], v. 4, n. 3, p. 120–126, 2013. Disponível em: https://emj.enu.kz/index.php/main/article/view/559. Acesso em: 27 aug. 2025.