@article{IM_1985__81_143253,
     author = {Lawrence. Corwin and B. Helffer},
     title = {Smoothness and analycity for solutions of first order systems of partial differential equations on nilpotent {Lie} groups.},
     journal = {Inventiones mathematicae},
     pages = {205--216},
     publisher = {mathdoc},
     volume = {81},
     year = {1985},
     zbl = {0588.35087},
     url = {http://geodesic.mathdoc.fr/item/IM_1985__81_143253/}
}