@article{ZNSL_1998_254_a12,
     author = {N. A. Shirokov},
     title = {Constructive descriptions of classes of functions with the help of polynomial {approximations.~I}},
     journal = {Zapiski Nauchnykh Seminarov POMI},
     pages = {207--234},
     year = {1998},
     volume = {254},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_1998_254_a12/}
}