@article{IM2_2007_71_1_a7,
     author = {T. Yu. Semenova},
     title = {Approximation by step functions of functions belonging to {Sobolev} spaces
},
     journal = {Izvestiya. Mathematics },
     pages = {149--180},
     publisher = {mathdoc},
     volume = {71},
     number = {1},
     year = {2007},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/IM2_2007_71_1_a7/}
}