@article{MZ_1967__98_170757,
     author = {Johann Walter},
     title = {Symmetrie elliptischer {Differentialoperatoren.}},
     journal = {Mathematische Zeitschrift},
     pages = {401--406},
     publisher = {mathdoc},
     volume = {98},
     year = {1967},
     zbl = {0146.34302},
     url = {http://geodesic.mathdoc.fr/item/MZ_1967__98_170757/}
}