The ratio spectrum of Lagrange constants under linear fractional transformations

Abstract

In this note we solve a problem posed by Lagarias and Shallit concerning Lagrange constants under linear fractional transformations $Mx=\frac{ax+b}{cx+d}$. For an integer matrix $M$ with nonzero determinant and relatively prime entries, define the ratio spectrum \begin{equation*} \mathcal{V}(M)=\left\{\frac{k(Mx)}{k(x)}:x\in\mathrm{Bad}\right\}, \end{equation*} where $k(x)$ denotes the Lagrange constant of the irrational number $x$ and $\mathrm{Bad}$ is the set of badly approximable numbers. Lagarias and Shallit proved that \begin{equation*} \mathcal{V}(M)\subseteq\left[\frac{1}{|\det M|},|\det M|\right], \end{equation*} and asked for the determination of $\mathcal{V}(M)$. We prove that \begin{equation*} \mathcal{V}(M)=\left[\frac{1}{|\det M|},|\det M|\right]. \end{equation*}