@article{SM_1976_29_4_a6,
     author = {A. A. Agrachev and R. V. Gamkrelidze},
     title = {A~second order optimality principle for a~time-optimal problem},
     journal = {Sbornik. Mathematics},
     pages = {547--576},
     publisher = {mathdoc},
     volume = {29},
     number = {4},
     year = {1976},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1976_29_4_a6/}
}