The algebraic modular functor conjecture in type $A_n$ quantum Teichmüller theory

Gus Schrader; Alexander Shapiro

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arXiv · 2509.03820

The algebraic modular functor conjecture in type $A_n$ quantum Teichmüller theory

Gus Schrader·Alexander Shapiro

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Abstract

Fock and Goncharov introduced a quantization of higher Teichmüller theory using cluster Poisson varieties and their noncommutative deformations, associating to a complex semisimple Lie group $G$ and a marked surface $S$ a quantum algebra $\mathbb{L}_{G,S}$ equipped with an action of the surface mapping class group. They conjectured that these quantizations form an algebraic analog of a modular functor: cutting a surface along a simple closed curve should correspond to a canonical gluing isomorphism for the associated algebras. In this paper we prove this conjecture for $G = \mathrm{PGL}_{n+1}$. Our approach requires two extensions of the Fock-Goncharov framework: (1) enhanced moduli spaces incorporating additional boundary data, providing algebro-geometric analogs of Fenchel-Nielsen twist coordinates; and (2) the residue universal Laurent ring, a refinement of the quantum universal Laurent ring obtained by localizing and imposing residue conditions. Using these tools, we construct canonical cutting isomorphisms that are equivariant under mapping class group actions and suffice to reconstruct the entire algebra $\mathbb{L}_{G,S}$ from data associated to the cut surface.

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Gus Schrader, Alexander Shapiro. 2025-09-04. The algebraic modular functor conjecture in type $A_n$ quantum Teichmüller theory. https://arxiv.org/abs/2509.03820

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