@article{MZM_1980_27_5_a1,
     author = {V. F. Babenko and S. A. Pichugov},
     title = {Best linear approximation of some classes of differentiable periodic functions},
     journal = {Matemati\v{c}eskie zametki},
     pages = {683--689},
     year = {1980},
     volume = {27},
     number = {5},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1980_27_5_a1/}
}