@article{MZM_1991_49_6_a6,
     author = {N. M. Isakov},
     title = {$C^r$-property and approximately differentiable functions},
     journal = {Matemati\v{c}eskie zametki},
     pages = {59--62},
     publisher = {mathdoc},
     volume = {49},
     number = {6},
     year = {1991},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1991_49_6_a6/}
}