@article{RCM_1968__2_181554,
     author = {N. Allan},
     title = {Some non-maximal arithmetic groups},
     journal = {Revista colombiana de matematicas},
     pages = {21--28},
     year = {1968},
     volume = {2},
     zbl = {0184.24403},
     url = {http://geodesic.mathdoc.fr/item/RCM_1968__2_181554/}
}