@article{MZ_1968__106_171012,
     author = {Johann Walter},
     title = {Symmetrie elliptischer {Differentialoperatoren.}},
     journal = {Mathematische Zeitschrift},
     pages = {149--152},
     year = {1968},
     volume = {106},
     zbl = {0159.40202},
     url = {http://geodesic.mathdoc.fr/item/MZ_1968__106_171012/}
}