@article{MS2_1962__10_165791,
     author = {Leon Greenberg},
     title = {Discrete {Subgroups} of the {Lorentz} {Group.}},
     journal = {Mathematica Scandinavica},
     pages = {85--107},
     year = {1962},
     volume = {10},
     zbl = {0118.03902},
     url = {http://geodesic.mathdoc.fr/item/MS2_1962__10_165791/}
}