@article{JRAM_1975__273_151535,
     author = {Alexander J. Hahn},
     title = {On the isomorphisms of the projective orthogonal groups and their congruence subgroups.},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {1--22},
     year = {1975},
     volume = {273},
     zbl = {0314.20032},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1975__273_151535/}
}