@article{MZ_1996__223_3_174921,
     author = {Harold Donnelly},
     title = {Elliptic operators and covers of {Riemannian} manifolds.},
     journal = {Mathematische Zeitschrift},
     pages = {303--308},
     publisher = {mathdoc},
     volume = {223},
     number = {3},
     year = {1996},
     zbl = {0871.58075},
     url = {http://geodesic.mathdoc.fr/item/MZ_1996__223_3_174921/}
}