@article{MZM_1989_46_1_a8,
     author = {O. A. Timoshin},
     title = {Best approximation of partial differential operators},
     journal = {Matemati\v{c}eskie zametki},
     pages = {78--87},
     publisher = {mathdoc},
     volume = {46},
     number = {1},
     year = {1989},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1989_46_1_a8/}
}