[1]
M. S. . Shabozov and M. O. Akobirshoev, “Mean-square approximation by an angle in $L_2$ and the values of quasiwidths for some classes of functions”, Ukr. Mat. Zhurn., vol. 72, no. 6, pp. 852–864, Jun. 2020, doi: 10.37863/umzh.v72i6.1064.