On the subgroups and normal subgroups of the group representation of the   Cayley tree
  In this paper, we give a characterization of the normal subgroups of index $2^{s}(2n+1),\ s\in\{1,2\},\ n\in \mathbb{N}$ and of the subgroups of index three of the group representation of the Cayley tree. 
F. H. Haydarov
math.GR