@misc{ZNSL_1978_81_a6,
     title = {Chapter~6. {Approximation}},
     journal = {Zapiski Nauchnykh Seminarov POMI},
     pages = {169--170},
     year = {1978},
     volume = {81},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_1978_81_a6/}
}