The ratio spectrum of Lagrange constants under linear fractional transformations

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$\mathrm{B}\mathrm{a}\mathrm{d}$\right}, \end{equation*} where $k(x)$ denotes the Lagrange constant of the irrational number $x$ and $\mathrm{B}\mathrm{a}\mathrm{d}$ 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*}