@article{MZM_1992_52_2_a5,
     author = {N. A. Il'yasov},
     title = {An inverse theorem of approximation theory of periodic functions in various metrics},
     journal = {Matemati\v{c}eskie zametki},
     pages = {53--61},
     publisher = {mathdoc},
     volume = {52},
     number = {2},
     year = {1992},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1992_52_2_a5/}
}