A weak Zassenhaus lemma for discrete subgroups of Diff(I)
  We prove a weaker version of Zassenhaus Lemma (also known as Margulis Lemma) for subgroups of Diff(I). We also show that a group with commutator subgroup containing a free subsemigroup does not admit a C_0-discrete faithful representation in Diff(I). 
Azer Akhmedov
math.GR math.DS