@article{MZ_1989__201_4_174034,
     author = {Rainer Nagel},
     title = {Towards a },
     journal = {Mathematische Zeitschrift},
     pages = {57--68},
     publisher = {mathdoc},
     volume = {201},
     number = {4},
     year = {1989},
     zbl = {0672.47001},
     url = {http://geodesic.mathdoc.fr/item/MZ_1989__201_4_174034/}
}