A sequence of algebraic integer relation numbers which converges to 4

Abstract

Let $\alpha \in \mathbb{R}$ and let $$A=\begin{bmatrix} 1 & 1 \\ 0 & 1\end{bmatrix} \ \text{and} \ B_{\alpha} = \begin{bmatrix} 1 & 0 \\ \alpha & 1\end{bmatrix}.$$ The subgroup $G_\alpha$ of $\mathrm{SL}_2(\mathbb{R})$ is a group generated by the matrices $A$ and $B_\alpha$. In this paper, we investigate the property of the group $G_\alpha.$ We construct a generalization of the Farey graph for the subgroup $G_\alpha.$ This graph determines whether the group $G_\alpha$ is a free group of rank $2$. More precisely, the group $G_\alpha$ is a free group of rank $2$ if and only if the graph is tree. In particular, we show that if $1/2$ is a vertex of the graph, then $G_\alpha$ is not a free group of rank $2$. Using this, we construct a sequence of real numbers so that the sequence converges to $4$ and each number has the corresponding group that is not a free group of rank $2$. It turns out that the real numbers are algebraic integers.