@article{JEMS_2009_11_4_a2,
     author = {Annalisa Baldi and Bruno Franchi and Maria Carla Tesi},
     title = {Hypoellipticity, fundamental solution and {Liouville} type theorem for matrix-valued differential operators in {Carnot} groups},
     journal = {Journal of the European Mathematical Society},
     pages = {777--798},
     publisher = {mathdoc},
     volume = {11},
     number = {4},
     year = {2009},
     doi = {10.4171/jems/166},
     url = {http://geodesic.mathdoc.fr/articles/10.4171/jems/166/}
}