Eigenfunctions of the Laplacian and associated Ruelle operator

Abstract

Let $Γ$ be a co-compact Fuchsian group of isometries on the Poincaré disk $\DD$ and $Δ$ the corresponding hyperbolic Laplace operator. Any smooth eigenfunction $f$ of $Δ$, equivariant by $Γ$ with real eigenvalue $λ=-s(1-s)$, where $s={1/2}+ it$, admits an integral representation by a distribution $\dd_{f,s}$ (the Helgason distribution) which is equivariant by $Γ$ and supported at infinity $\partial\DD=\SS^1$. The geodesic flow on the compact surface $\DD/Γ$ is conjugate to a suspension over a natural extension of a piecewise analytic map $T:\SS^1\to\SS^1$, the so-called Bowen-Series transformation. Let $\ll_s$ be the complex Ruelle transfer operator associated to the jacobian $-s\ln |T'|$. M. Pollicott showed that $\dd_{f,s}$ is an eigenfunction of the dual operator $\ll_s^*$ for the eigenvalue 1. Here we show the existence of a (nonzero) piecewise real analytic eigenfunction $ψ_{f,s}$ of $\ll_s$ for the eigenvalue 1, given by an integral formula \[ ψ_{f,s} (ξ)=\int \frac{J(ξ,η)}{|ξ-η|^{2s}} \dd_{f,s} (dη), \] \noindent where $J(ξ,η)$ is a $\{0,1\}$-valued piecewise constant function whose definition depends upon the geometry of the Dirichlet fundamental domain representing the surface $\DD/Γ$.