@article{CMH_1970__45_139435,
     author = {Peter Hess},
     title = {Zur {St\"orungstheorie} linearer {Operatoren} in {Banachr\"aumen.}},
     journal = {Commentarii mathematici Helvetici},
     pages = {229--235},
     publisher = {mathdoc},
     volume = {45},
     year = {1970},
     zbl = {0188.44501},
     url = {http://geodesic.mathdoc.fr/item/CMH_1970__45_139435/}
}