@article{MAN_1950__122_160226,
     author = {Franz Rellich},
     title = {Halbbeschr\"ankte gew\"ohnliche {Differentialoperatoren} zweiter {Ordnung}},
     journal = {Mathematische Annalen},
     pages = {343--368},
     publisher = {mathdoc},
     volume = {122},
     year = {1950},
     zbl = {0044.31201},
     url = {http://geodesic.mathdoc.fr/item/MAN_1950__122_160226/}
}