@article{DAN_1967_177_2_a3,
     author = {P. P. Korovkin},
     title = {An extermal problem and linear methods of summation of {Fourier} series},
     journal = {Doklady Akademii Nauk},
     pages = {265--267},
     publisher = {mathdoc},
     volume = {177},
     number = {2},
     year = {1967},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1967_177_2_a3/}
}