A composition law and refined notions of convergence for periodic continued fractions

Abstract

We define an equivalence relation on periodic continued fractions with partial quotients in a ring $\mathcal{O} \subseteq \mathbf{C}$, a group law on these equivalence classes, and a map from these equivalence classes to matrices in $\mathrm{GL}_2(\mathcal{O})$ with determinant $\pm1$. We prove this group of equivalence classes is isomorphic to $\mathbf{Z}/2\mathbf{Z}\ast\mathcal{O}$ and study certain of its one- and two-dimensional representations. For a periodic continued fraction with period $k$, we give a refined description of the limits of the $k$ different $k$-decimations of its sequence of convergents. We show that for a periodic continued fraction associated to a matrix with eigenvalues of different magnitudes, all $k$ of these limits exist in $\mathbb{P}^1(\mathbf{C})$ and a strict majority of them are equal.