@article{MZM_1968_3_2_a4,
     author = {Yu. N. Subbotin and L. V. Taikov},
     title = {Best approximation of a~differentiation operator in $L_2$-space},
     journal = {Matemati\v{c}eskie zametki},
     pages = {157--164},
     publisher = {mathdoc},
     volume = {3},
     number = {2},
     year = {1968},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1968_3_2_a4/}
}