@article{TM_1987_180_a53,
     author = {A. N. Davidchik and A. A. Ligun},
     title = {Approximation of periodic functions by linear operators},
     journal = {Trudy Matematicheskogo Instituta imeni V.A. Steklova},
     pages = {98--99},
     publisher = {mathdoc},
     volume = {180},
     year = {1987},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/TM_1987_180_a53/}
}