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    • Free subgroups of the parametrized modular group 

      Pommerenke, Christian; Toro, Margarita
      We study free subgroups of index four of the parametrized modular group Π, the subgroup of SL(2; Z[ξ]) generated by (10 ξ1) and (01 -10). There are eight free subgroups, four of which are normal and four are non-normal. ...
      Universidad Nacional de Colombia Revistas electrónicas UN Revista Colombiana de Matemáticas. 
    • On the two-parabolic subgroups of sl(2,c) 

      Pommerenke, Christian; Toro, Margarita
      We consider homomorphisms $H_{t}$ from the free group $F$ of rank $2$ onto the subgroup of SL$(2,\mathbb{C})$ that is generated by two parabolic matrices. Up to conjugation, $H_{t}$ depends only on one complex parameter ...
      Universidad Nacional de Colombia Revistas electrónicas UN Revista Colombiana de Matemáticas.