Geodesic continued fraction for Shimura curves and its periodicity: the case of $(2,3,7)$-triangle group

In this paper we study the geodesic continued fraction in the case of the Shimura curve coming from the $(2,3,7)$-triangle group. We construct a certain continued fraction expansion of real numbers using the so-called coding of the geodesics on the Shimura curve, and prove the Lagrange type periodicity theorem for the expansion which captures the fundamental relative units of quadratic extensions of $\mathbb{Q}(\cos (2\pi \mathrm{/}7))$ with rank one relative unit groups. We also discuss the convergence of these continued fractions.