@article{DE_1971_7_8_a4,
     author = {Yu. N. Kiselev},
     title = {Differentiability of a mapping that describes isochrone surfaces in a linear time optimal problem},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1385--1392},
     publisher = {mathdoc},
     volume = {7},
     number = {8},
     year = {1971},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1971_7_8_a4/}
}