@article{MZM_1990_48_4_a12,
     author = {O. A. Timoshin},
     title = {Order of the best approximation of partial differential operators},
     journal = {Matemati\v{c}eskie zametki},
     pages = {115--121},
     publisher = {mathdoc},
     volume = {48},
     number = {4},
     year = {1990},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1990_48_4_a12/}
}