@article{IM2_1981_17_3_a6,
     author = {E. A. Sevast'yanov},
     title = {The degree of rational approximation of functions and their differentiability},
     journal = {Izvestiya. Mathematics},
     pages = {595--600},
     year = {1981},
     volume = {17},
     number = {3},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/IM2_1981_17_3_a6/}
}