@article{10_4153_CMB_1994_045_4,
     author = {Bergamasco, Adalberto P. and Zani, S\'ergio Lu{\'\i}s},
     title = {Global {Hypoellipticity} of a {Class} of {Second} {Order} {Operators}},
     journal = {Canadian mathematical bulletin},
     pages = {301--305},
     year = {1994},
     volume = {37},
     number = {3},
     doi = {10.4153/CMB-1994-045-4},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CMB-1994-045-4/}
}