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@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/} }
Jöran Friberg. Partially Hypoelliptic Differential Equations of Finite Type.. Mathematica Scandinavica, Tome 9 (1961), pp. 22-42. http://geodesic.mathdoc.fr/item/MS2_1961__9_165760/