@article{MZ_1984__187_173491,
     author = {C. Robin Graham},
     title = {Compatibility {Operators} for {Degenerate} {Elliptic} {Equations} on the {Ball} and {Heisenberg} {Groups.}},
     journal = {Mathematische Zeitschrift},
     pages = {289--304},
     publisher = {mathdoc},
     volume = {187},
     year = {1984},
     zbl = {0538.47031},
     url = {http://geodesic.mathdoc.fr/item/MZ_1984__187_173491/}
}