Ratio coordinates for higher Teichmüller spaces

Abstract

We define new coordinates for Fock–Goncharov’s higher Teichmüller spaces for a surface with holes, which are the moduli spaces of representations of the fundamental group into a reductive Lie group G. Some additional data on the boundary leads to two closely related moduli spaces, the $\mathscr {X}$-space and the $\mathscr {A}$-space, forming a cluster ensemble. Fock and Goncharov gave nice descriptions of the coordinates of these spaces in the cases of $G = \mathrm{PGL}_m$ and $G=\mathrm{SL}_m$, together with Poisson structures. We consider new coordinates for higher Teichmüller spaces given as ratios of the coordinates of the $\mathscr {A}$-space for $G=\mathrm{SL}_m$, which are generalizations of Kashaev’s ratio coordinates in the case $m=2$. Using Kashaev’s quantization for $m=2$, we suggest a quantization of the system of these new ratio coordinates, which may lead to a new family of projective representations of mapping class groups. These ratio coordinates depend on the choice of an ideal triangulation decorated with a distinguished corner at each triangle, and the key point of the quantization is to guarantee certain consistency under a change of such choices. We prove this consistency for $m=3$, and for completeness we also give a full proof of the presentation of Kashaev’s groupoid of decorated ideal triangulations.