@article{GDZPPN002429101, title = {Positive Operators and Elliptic Eigenvalue Problems.}, journal = {Mathematische Zeitschrift}, volume = {185}, year = {1984}, pages = {247-264}, zbl = {0549.47026}, url = {http://dml.mathdoc.fr/item/GDZPPN002429101} }
Nussbaum, Roger D. Positive Operators and Elliptic Eigenvalue Problems.. Mathematische Zeitschrift, Tome 185 (1984) pp. 247-264. http://gdmltest.u-ga.fr/item/GDZPPN002429101/