@article{IM_1991__106_1_143929,
     author = {M. Burger and P. Sarnak},
     title = {Ramanujan duals {II.}},
     journal = {Inventiones mathematicae},
     pages = {1--12},
     publisher = {mathdoc},
     volume = {106},
     number = {1},
     year = {1991},
     zbl = {0774.11021},
     url = {http://geodesic.mathdoc.fr/item/IM_1991__106_1_143929/}
}