@article{JRAM_1963__212_150568,
     author = {Hans Petersson},
     title = {\"Uber die {Kongruenzgruppen} der {Stufe} 4.},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {63--72},
     year = {1963},
     volume = {212},
     zbl = {0111.27703},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1963__212_150568/}
}