@article{MZ_1976__148_172371,
     author = {Peter Volkmann and Helmut Kroh},
     title = {St\"orungss\"atze f\"ur {Semifredholmoperatoren.}},
     journal = {Mathematische Zeitschrift},
     pages = {295--298},
     year = {1976},
     volume = {148},
     zbl = {0318.47008},
     url = {http://geodesic.mathdoc.fr/item/MZ_1976__148_172371/}
}