@article{JRAM_1995__464_153730,
     author = {F. Ricci and F. De Mari and M.M. Peloso},
     title = {Analysis of second order differential operators with complex coefficients on the {Heisenberg} group.},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {67--96},
     publisher = {mathdoc},
     volume = {464},
     year = {1995},
     zbl = {0826.43004},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1995__464_153730/}
}