@article{IM2_1971_5_1_a7,
     author = {N. P. Korneichuk},
     title = {Extreme values of functionals and best approximation on classes of periodic functions},
     journal = {Izvestiya. Mathematics },
     pages = {97--129},
     publisher = {mathdoc},
     volume = {5},
     number = {1},
     year = {1971},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/IM2_1971_5_1_a7/}
}