@article{RM_1964_19_2_a7,
     author = {I. M. Petrov},
     title = {On the order of approximation of functions of class $Z_1$ by positive linear polynomial operators},
     journal = {Trudy Matematicheskogo Instituta imeni V.A. Steklova},
     publisher = {mathdoc},
     volume = {19},
     number = {2},
     year = {1964},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/RM_1964_19_2_a7/}
}