Trace Minimization and Roots in ${\rm PSL}(2,\mathbb{R})$

Abstract

Suppose that $A,B \in {\rm PSL}(2,\mathbb{R})$ generate a non-elementary Fuchsian group. Let $m,n\in\mathbb{N}_+$, and let $R,S\in {\rm PSL}(2,\mathbb{R})$ such that $R^m=A$ and $S^n=B$. We present explicit algorithms to check whether $\langle R,S\rangle$ is a Fuchsian group. These algorithms rely only on the knowledge of the traces ${\rm tr}(A)$, ${\rm tr}(B)$, and ${\rm tr}(AB)$, which we assume to be given as algebraic numbers. The main tools are the classic Trace Minimization Algorithm, as introduced in 1972 by the third author, a new Extended Trace Minimization Algorithm, and a Rational Angle Recovery Algorithm which checks whether a given number $x$ is if the form $x = 2 \cos(p \pi/q)$. The question when roots of the generators of a free Fuchsian group of rank 2 generate again a free Fuchsian group of rank 2, and an extension to positive rational exponents $m,n$ are treated, as well.