@article{MZM_1976_19_3_a4,
     author = {V. Yu. Popov},
     title = {Direct and inverse inequalities for $\varphi${-Fej\'er} mean-square approximations},
     journal = {Matemati\v{c}eskie zametki},
     pages = {353--364},
     publisher = {mathdoc},
     volume = {19},
     number = {3},
     year = {1976},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1976_19_3_a4/}
}