@article{MS2_1961__9_165760,
     author = {J\"oran Friberg},
     title = {Partially {Hypoelliptic} {Differential} {Equations} of {Finite} {Type.}},
     journal = {Mathematica Scandinavica},
     pages = {22--42},
     publisher = {mathdoc},
     volume = {9},
     year = {1961},
     zbl = {0108.10201},
     url = {http://geodesic.mathdoc.fr/item/MS2_1961__9_165760/}
}