@article{IM_1984__78_143171,
     author = {Antonio S\'anchez-Calle},
     title = {Fundamental solutions and geometry of the sum of squares of vector fields.},
     journal = {Inventiones mathematicae},
     pages = {143--160},
     publisher = {mathdoc},
     volume = {78},
     year = {1984},
     zbl = {0582.58004},
     url = {http://geodesic.mathdoc.fr/item/IM_1984__78_143171/}
}