Ruijsenaars wavefunctions as modular group matrix coefficients

We give a description of the Hallnäs–Ruijsenaars eigenfunctions of the 2-particle hyperbolic Ruijsenaars system as matrix coefficients for the order 4 element S∈SL(2,Z) acting on the Hilbert space of GL(2) quantum Teichmüller theory on the punctured torus. The GL(2) Macdonald polynomials are then obtained as special values of the analytic continuation of these matrix coefficients. The main tool used in the proof is the cluster structure on the moduli space of framed GL(2)-local systems on the punctured torus, and an SL(2,Z)-equivariant embedding of the GL(2) spherical DAHA into the quantized coordinate ring of the corresponding cluster Poisson variety.