@article{MAN_1975__217_162755,
     author = {Werner Stork},
     title = {Selbstadjungierte {Differentialoperatoren} erster {Ordnung} in {A2} {(Co).}},
     journal = {Mathematische Annalen},
     pages = {69--80},
     publisher = {mathdoc},
     volume = {217},
     year = {1975},
     zbl = {0308.47038},
     url = {http://geodesic.mathdoc.fr/item/MAN_1975__217_162755/}
}