@article{JRAM_1989__398_153139,
     author = {W. Allegretto and A.B. Mingarelli},
     title = {Boundary problems of the second order with an indefinite weight-function.},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {1--24},
     publisher = {mathdoc},
     volume = {398},
     year = {1989},
     zbl = {0668.35073},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1989__398_153139/}
}