@article{IM_1991__104_1_143888,
     author = {J. Nourrigat},
     title = {Syst\`eme sous-elliptiques {II.}},
     journal = {Inventiones mathematicae},
     pages = {377--400},
     year = {1991},
     volume = {104},
     number = {1},
     zbl = {0771.35086},
     url = {http://geodesic.mathdoc.fr/item/IM_1991__104_1_143888/}
}