@article{MZ_1984__186_173442,
     author = {Roger D. Nussbaum},
     title = {Positive {Operators} and {Elliptic} {Eigenvalue} {Problems.}},
     journal = {Mathematische Zeitschrift},
     pages = {247--264},
     publisher = {mathdoc},
     volume = {186},
     year = {1984},
     zbl = {0549.47026},
     url = {http://geodesic.mathdoc.fr/item/MZ_1984__186_173442/}
}