@article{MAN_1966__163_161367,
     author = {S. HILDEBRANDT},
     title = {\"Uber den numerischen {Wertebereich} eines {Operators.}},
     journal = {Mathematische Annalen},
     pages = {230--247},
     publisher = {mathdoc},
     volume = {163},
     year = {1966},
     zbl = {0138.39101},
     url = {http://geodesic.mathdoc.fr/item/MAN_1966__163_161367/}
}