@article{JRAM_1972__257_151268,
     author = {Robert M. Kauffman},
     title = {Compactness of the inverse of the minimal operator for a class of ordinary differential expressions.},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {91--99},
     publisher = {mathdoc},
     volume = {257},
     year = {1972},
     zbl = {0248.34035},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1972__257_151268/}
}