@article{10_1090_S1079_6762_03_00107_0,
     author = {Bonfiglioli, Andrea and Lanconelli, Ermanno and Uguzzoni, Francesco},
     title = {Levi{\textquoteright}s parametrix for some sub-elliptic non-divergence form operators},
     journal = {Electronic research announcements of the American Mathematical Society},
     pages = {10--18},
     year = {2003},
     volume = {09},
     doi = {10.1090/S1079-6762-03-00107-0},
     url = {http://geodesic.mathdoc.fr/articles/10.1090/S1079-6762-03-00107-0/}
}