Quotients of trees for arithmetic subgroups of PGL2 over a rational   function field
  In this note we compute the fundamental domain of the action on the corresponding Bruhat-Tits tree of the arithmetic subgroup of PGL2 with respect to an arbitrary place of the rational function field. 
Ralf K\"ohl and Bernhard M\"uhlherr and Koen Struyve
math.GR