The problem of freeness for Euler monoids and Möbius groups
The authors extend the set of those rational numbers \(\mu\in(-2,2)\) for which the group\break \(\Bigl\langle\left(\begin{smallmatrix} 1 &\mu\\ 0 &1\end{smallmatrix}\right),\left(\begin{smallmatrix} 1 &0\\ \mu &1\end{smallmatrix}\right)\Bigr\rangle\) is not free.
scientific article; zbMATH DE number 1523493