@article{CLTM_1971_1971_1_a2,
     author = {Lempert, Heinrich Kurt},
     title = {Zur {Storungstheorie} endlich-vielfacher {Eigenwerte.}},
     journal = {Collectanea mathematica},
     pages = {19-50},
     publisher = {mathdoc},
     volume = {22},
     number = {1},
     year = {1971},
     zbl = {0228.47011},
     url = {http://geodesic.mathdoc.fr/item/CLTM_1971_1971_1_a2/}
}