@article{MZM_1981_29_4_a10,
     author = {A. V. Kolushov},
     title = {Differentiability of the best approximation operator},
     journal = {Matemati\v{c}eskie zametki},
     pages = {577--596},
     year = {1981},
     volume = {29},
     number = {4},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1981_29_4_a10/}
}