@article{SM_1988_60_1_a14,
     author = {A. I. Tret'yak},
     title = {On~necessary conditions of arbitrary order for optimality in a~time-optimal problem},
     journal = {Sbornik. Mathematics},
     pages = {255--268},
     publisher = {mathdoc},
     volume = {60},
     number = {1},
     year = {1988},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1988_60_1_a14/}
}