@article{10_4171_ggd_110,
     author = {Volodymyr V. Nekrashevych},
     title = {Free subgroups in groups acting on rooted trees},
     journal = {Groups, geometry, and dynamics},
     pages = {847--862},
     year = {2010},
     volume = {4},
     number = {4},
     doi = {10.4171/ggd/110},
     url = {http://geodesic.mathdoc.fr/articles/10.4171/ggd/110/}
}