@article{DAN_1957_116_5_a7,
     author = {V. I. Plotnikov},
     title = {On the differentiability of the solutions of variation problems in a non-parametric form},
     journal = {Doklady Akademii Nauk},
     pages = {746--749},
     publisher = {mathdoc},
     volume = {116},
     number = {5},
     year = {1957},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1957_116_5_a7/}
}