A Note on Discrete Groups
  We prove that a group has a DF domain if and only if has a Double Dirichlet domain. We give a good describtion of the side-paring transformation of such groups. In particular, those not fixing infinity have a common eigenvector. We also give other results on Kleinian and Fuchsian groups. 
Stanley O. Juriaans, Severino C. Lima Neto, Antonio de A. E. Silva
math.GR math.NT math.RA