@article{IM_1993__114_1_144154,
     author = {Robert L. Bryant and Lucas Hsu},
     title = {Rigidity of integral curves of rank 2 distributions.},
     journal = {Inventiones mathematicae},
     pages = {435--462},
     publisher = {mathdoc},
     volume = {114},
     number = {1},
     year = {1993},
     zbl = {0807.58007},
     url = {http://geodesic.mathdoc.fr/item/IM_1993__114_1_144154/}
}