@article{MZ_1957__68_169712,
     author = {G\"unter Hellwig},
     title = {\"Uber {Systeme} hyperbolischer {Differentialgleichungen} erster {Ordnung.} {III.}},
     journal = {Mathematische Zeitschrift},
     pages = {325--337},
     year = {1957},
     volume = {68},
     zbl = {0142.37002},
     url = {http://geodesic.mathdoc.fr/item/MZ_1957__68_169712/}
}