@article{MZM_1967_1_2_a1,
     author = {S. B. Stechkin},
     title = {Best approximation of linear operators},
     journal = {Matemati\v{c}eskie zametki},
     pages = {137--148},
     publisher = {mathdoc},
     volume = {1},
     number = {2},
     year = {1967},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1967_1_2_a1/}
}