@article{10_4064_ap_17_1_79_115,
     author = {W. D\"uck},
     title = {Variationsprinzipien bei nat\"urlichen {Eigenwertproblemen}},
     journal = {Annales Polonici Mathematici},
     pages = {79--115},
     publisher = {mathdoc},
     volume = {17},
     number = {1},
     year = {1965},
     doi = {10.4064/ap-17-1-79-115},
     language = {de},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/ap-17-1-79-115/}
}