@article{10_4171_ggd_38,
     author = {Christian Liedtke},
     title = {Natural central extensions of groups},
     journal = {Groups, geometry, and dynamics},
     pages = {245--261},
     year = {2008},
     volume = {2},
     number = {2},
     doi = {10.4171/ggd/38},
     url = {http://geodesic.mathdoc.fr/articles/10.4171/ggd/38/}
}