@article{MAN_1957__133_160546,
     author = {F.W. Sch\"afke},
     title = {Zur {St\"orungstheorie} der {Spektralzerlegung.}},
     journal = {Mathematische Annalen},
     pages = {219--234},
     publisher = {mathdoc},
     volume = {133},
     year = {1957},
     zbl = {0081.12101},
     url = {http://geodesic.mathdoc.fr/item/MAN_1957__133_160546/}
}