@article{MM2_1974__14_154285,
     author = {Wilhelm Singhof},
     title = {\"Uber nichtlineare {Spektral-} und {St\"orungstheorie.}},
     journal = {Manuscripta mathematica},
     pages = {123--162},
     publisher = {mathdoc},
     volume = {14},
     year = {1974},
     zbl = {0295.47064},
     url = {http://geodesic.mathdoc.fr/item/MM2_1974__14_154285/}
}