@article{10_4171_ggd_126,
     author = {Bachir Bekka and Alexander Lubotzky},
     title = {Lattices with  and lattices without spectral gap},
     journal = {Groups, geometry, and dynamics},
     pages = {251--264},
     year = {2011},
     volume = {5},
     number = {2},
     doi = {10.4171/ggd/126},
     url = {http://geodesic.mathdoc.fr/articles/10.4171/ggd/126/}
}