@article{GDZPPN002107740,
title = {Analysis of second order differential operators on Heisenberg groups. I.},
journal = {Inventiones mathematicae},
volume = {100},
year = {1990},
pages = {545-582},
zbl = {0742.43006},
url = {http://dml.mathdoc.fr/item/GDZPPN002107740}
}
Müller, D.; Ricci, F. Analysis of second order differential operators on Heisenberg groups. I.. Inventiones mathematicae, Tome 100 (1990) pp. 545-582. http://gdmltest.u-ga.fr/item/GDZPPN002107740/