@article{SM_2018_209_11_a0,
     author = {A. A. Ardentov and Yu. L. Sachkov and T. Huang and X. Yang},
     title = {Extremal trajectories in the {sub-Lorentzian} problem on the {Engel} group},
     journal = {Sbornik. Mathematics},
     pages = {1547--1574},
     publisher = {mathdoc},
     volume = {209},
     number = {11},
     year = {2018},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_2018_209_11_a0/}
}