@article{MAN_1996__304_4_165414,
     author = {Fulvio Ricci and Detlef M\"uller},
     title = {Solvability for a class of non-homogeneous differential operators on two-step nilpotent groups.},
     journal = {Mathematische Annalen},
     pages = {517--548},
     publisher = {mathdoc},
     volume = {304},
     number = {4},
     year = {1996},
     zbl = {0848.43008},
     url = {http://geodesic.mathdoc.fr/item/MAN_1996__304_4_165414/}
}