• One-cusped congruence subgroups of PSL₂ (Ok) 

      Petersen, Kathleen Lizabeth (2005)
      Let K be a number field with r real places and s complex places, and let OK be the ring of integers of K. The group PSL2(OK) embeds discretely in PSL2(R) r × PSL2(C) s , the group of orientation preserving isometries ...