@article{JEDP_2000____A15_0,
     author = {Ricci, Fulvio},
     title = {Solvability of second-order left-invariant differential operators on the {Heisenberg} group},
     journal = {Journ\'ees \'equations aux d\'eriv\'ees partielles},
     eid = {15},
     pages = {1--10},
     publisher = {Universit\'e de Nantes},
     year = {2000},
     mrnumber = {2002c:22018},
     zbl = {01808705},
     language = {en},
     url = {http://archive.numdam.org/item/JEDP_2000____A15_0/}
}