@article{MZ_1967__96_170670,
     author = {Johann Schr\"oder},
     title = {Zusammenh\"angende {Mengen} inverspositiver {Differentialoperatoren} vierter {Ordnung.}},
     journal = {Mathematische Zeitschrift},
     pages = {89--110},
     publisher = {mathdoc},
     volume = {96},
     year = {1967},
     zbl = {0148.06103},
     url = {http://geodesic.mathdoc.fr/item/MZ_1967__96_170670/}
}