@article{CMH_1965__40_139297,
     author = {Bruno Peter Zwahlen},
     title = {\"Uber die {Eigenwerte} der {Summe} zweier selbstadjungierter {Operatoren.}},
     journal = {Commentarii mathematici Helvetici},
     pages = {81--116},
     publisher = {mathdoc},
     volume = {40},
     year = {1965},
     zbl = {0135.36501},
     url = {http://geodesic.mathdoc.fr/item/CMH_1965__40_139297/}
}