@article{MZ_1950__53_169186,
     author = {W. und G. HAACK and G. HELLWIG},
     title = {\"Uber {Systeme} hyperbolischer {Differentialgleichun-gen} erster {Ordnung} {I}},
     journal = {Mathematische Zeitschrift},
     pages = {244--266},
     year = {1950},
     volume = {53},
     zbl = {0041.21602},
     url = {http://geodesic.mathdoc.fr/item/MZ_1950__53_169186/}
}