@article{MAN_1990__287_1_164715,
     author = {Robert J. Zimmer},
     title = {Unitary spectum and the fundamental group for actions of semisimple {Lie} groups.},
     journal = {Mathematische Annalen},
     pages = {697--702},
     publisher = {mathdoc},
     volume = {287},
     number = {1},
     year = {1990},
     zbl = {0681.22014},
     url = {http://geodesic.mathdoc.fr/item/MAN_1990__287_1_164715/}
}