@article{MAN_1978__234_163129,
     author = {Peter Volkmann},
     title = {Ein {St\"orungssatz} f\"ur abgeschlossene, lineare {Operatoren.}},
     journal = {Mathematische Annalen},
     pages = {139--144},
     publisher = {mathdoc},
     volume = {234},
     year = {1978},
     zbl = {0358.47008},
     url = {http://geodesic.mathdoc.fr/item/MAN_1978__234_163129/}
}