@article{MZ_1950__52_169122,
     author = {W. Haack},
     title = {Strahlenkomplexe mit integrablen {Pfaffschen} {Differentialformen}},
     journal = {Mathematische Zeitschrift},
     pages = {322--341},
     year = {1950},
     volume = {52},
     zbl = {0034.09102},
     url = {http://geodesic.mathdoc.fr/item/MZ_1950__52_169122/}
}