@article{JRAM_1965__220_150709,
     author = {Charles W. Curtis},
     title = {Central extensions of groups of {Lie} type.},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {174--185},
     publisher = {mathdoc},
     volume = {220},
     year = {1965},
     zbl = {0137.25701},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1965__220_150709/}
}