ShabozovM. S., & Akobirshoev M. O. (2020). Mean-square approximation by an angle in $L_2$ and the values of quasiwidths for some classes of functions. Ukrains’kyi Matematychnyi Zhurnal, 72(6), 852-864. https://doi.org/10.37863/umzh.v72i6.1064