Locally infinite graphs and symmetries

TL;DR

This paper investigates the properties of infinite, transitive graphs, especially Cayley graphs, exploring concepts like generalized diameter and local isomorphisms, and demonstrating that regularity properties of locally finite graphs do not always extend.

Contribution

It introduces the notion of generalized diameter for infinite graphs, constructs Cayley graphs with arbitrary generalized diameters, and shows that local isomorphism does not imply global isomorphism.

Findings

Existence of Cayley graphs with infinite diameter but no infinite geodesic rays.

Generalized diameter can be an ordinal or 'truly infinite', reflecting extension properties.

Construction of graphs with identical local neighborhoods but different global structures.

Abstract

When one studies geometric properties of graphs, local finiteness is a common implicit assumption, and that of transitivity a frequent explicit one. By compactness arguments, local finiteness guarantees several regularity properties. It is generally easy to find counterexamples to such regularity results when the assumption of local finiteness is dropped. The present work focuses on the following problem: determining whether these regularity properties still hold when local finiteness is replaced by an assumption of transitivity. After recalling the locally finite situation, we show that there are Cayley graphs with infinite generating systems that have infinite diameter but do not contain any infinite geodesic ray. We also introduce a notion of generalised diameter. The generalised diameter of a graph is either an ordinal or "truly infinite" and captures the extension properties of geodesic paths. It is a finite ordinal if and only if the usual diameter is finite, and in that case the two notions agree. Besides, the generalised diameter is "truly infinite" if and only if the considered graph contains an infinite geodesic ray. We show that there exist Cayley graphs of abelian groups of arbitrary generalised diameter. Finally, we build Cayley graphs of abelian groups that have isomorphic balls of radius R for every R but are not globally isomorphic. This enables us to construct a non-transitive graph such that for every R and any vertices u and v, the ball of centre u and radius R is isomorphic to that of centre v and same radius.