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@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/} }
Roger D. Nussbaum. Positive Operators and Elliptic Eigenvalue Problems.. Mathematische Zeitschrift, Tome 186 (1984), pp. 247-264. http://geodesic.mathdoc.fr/item/MZ_1984__186_173442/