@article{JRAM_1985__357_152710,
     author = {Robert Brooks},
     title = {The bottom of the spectrum of a {Riemannian} covering.},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {101--114},
     year = {1985},
     volume = {357},
     zbl = {0553.53027},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1985__357_152710/}
}