@article{MZM_1989_45_2_a4,
     author = {A. V. Kolushov},
     title = {Differential properties of the operator of best approximation by rational fractions},
     journal = {Matemati\v{c}eskie zametki},
     pages = {40--50},
     publisher = {mathdoc},
     volume = {45},
     number = {2},
     year = {1989},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1989_45_2_a4/}
}