# On Sullivan's construction of eigenfunctions via exit times of Brownian motion

## TL;DR

This paper details Sullivan's method of constructing Laplacian eigenfunctions on Riemannian manifolds using Brownian motion exit times, providing a probabilistic approach to spectral analysis.

## Contribution

It clarifies and elaborates on Sullivan's construction of eigenfunctions via exit times of Brownian motion on negatively curved manifolds.

## Key findings

- Eigenfunctions can be represented as expected values involving exit times
- The construction applies to manifolds with pinched negative curvature
- Eigenfunctions correspond to complex eigenvalues with real part greater than the spectrum's supremum

## Abstract

The purpose of this note is to give details for an argument of Sullivan to construct eigenfunctions of the Laplacian on a Riemannian manifold using exit times of Brownian motion \cite{sullivanpos}. Let $X$ be a complete, simply connected Riemannian manifold of pinched negative sectional curvature. Let $\lambda_1 = \lambda_1(X) < 0$ be the supremum of the spectrum of the Laplacian on $L^2(X)$, and let $D \subset X$ be a bounded domain in $X$ with smooth boundary. Let $(B_t)_{t \geq 0}$ be Brownian motion on $X$ and let $\tau = \tau_D$ be the first exit time of Brownian motion from $D$. For each $\lambda \in \mathbb{C}$ with $\hbox{Re } \ \lambda > \lambda_1$ and $x \in D$, we show that for any continuous function $\phi : \partial D \to \mathbb{C}$, the function $$ h(x) = \mathbb{E}_x(e^{-\lambda \tau} \phi(B_{\tau})) \ , \ x \in D, $$ is an eigenfunction of the Laplacian on $D$ with eigenvalue $\lambda$ and boundary value $\phi$.