Multiple boundary representations of $\lambda$-harmonic functions on trees

TL;DR

This paper develops a boundary integral representation for eigenfunctions of stochastic operators on trees, introducing multiple boundary representations and analyzing their properties, especially under group invariance and resolvent conditions.

Contribution

It introduces a framework for multiple boundary representations of $oldsymbol{ ext{λ}}$-harmonic functions on trees, including conditions for their existence and comparison, extending classical potential theory.

Findings

Boundary integral representation for eigenfunctions with complex eigenvalues.

Existence of multiple boundary representations under group invariance.

Conditions under which distributions extend to measures.

Abstract

We consider a countable tree $T$, possibly having vertices with infinite degree, and an arbitrary stochastic nearest neighbour transition operator $P$. We provide a boundary integral representation for general eigenfunctions of $P$ with eigenvalue $\lambda \in \mathbb{C}$, under the condition that the oriented edges can be equipped with complex-valued weights satisfying three natural axioms. These axioms guarantee that one can construct a $\lambda$-Poisson kernel. The boundary integral is with respect to distributions, that is, elements in the dual of the space of locally constant functions. Distributions are interpreted as finitely additive complex measures. In general, they do not extend to $\sigma$-additive measures: for this extension, a summability condition over disjoint boundary arcs is required. Whenever $\lambda$ is in the resolvent of $P$ as a self-adjoint operator on a naturally associated $\ell^2$-space and the diagonal elements of the resolvent (`Green function') do not vanish at $\lambda$, one can use the ordinary edge weights corresponding to the Green function and obtain the ordinary $\lambda$-Martin kernel. We then consider the case when $P$ is invariant under a transitive group action. In this situation, we study the phenomenon that in addition to the $\lambda$-Martin kernel, there may be further choices for the edge weights which give rise to another $\lambda$-Poisson kernel with associated integral representations. In particular, we compare the resulting distributions on the boundary. The material presented here is closely related to the contents of our `companion' paper arXiv:1802.01976