@article{IM2_1989_33_3_a8,
     author = {A. K. Kushpel'},
     title = {Sharp estimates of the widths of convolution classes},
     journal = {Izvestiya. Mathematics},
     pages = {631--649},
     year = {1989},
     volume = {33},
     number = {3},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/IM2_1989_33_3_a8/}
}