@article{UMJ_2015_1_1_a1,
     author = {Vitalii V. Arestov},
     title = {On the best approximation of the differentiation operator},
     journal = {Ural mathematical journal},
     pages = {20--29},
     year = {2015},
     volume = {1},
     number = {1},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/UMJ_2015_1_1_a1/}
}