Large-scale geometry of Borel graphs of polynomial growth

TL;DR

This paper investigates the large-scale geometry of Borel graphs with polynomial growth, establishing universal properties, embeddings, and structural results that connect descriptive set theory with geometric group theory.

Contribution

It generalizes known embeddings of polynomial growth graphs, proves Borel graphs of polynomial growth are hyperfinite, and introduces new methods using padded decompositions.

Findings

Graphs of polynomial growth admit coarse embeddings into ^n.

All Borel graphs of polynomial growth are hyperfinite.

Borel graphs of polynomial growth support toast structures.

Abstract

We study graphs of polynomial growth from the perspective of asymptotic geometry and descriptive set theory. The starting point of our investigation is a theorem of Krauthgamer and Lee who showed that every connected graph of polynomial growth admits an injective contraction mapping to $(\mathbb Z^n, \|\cdot\|_\infty)$ for some $n\in\mathbb N$. We strengthen and generalize this result in a number of ways. In particular, answering a question of Papasoglu, we construct coarse embeddings from graphs of polynomial growth to $\mathbb Z^n$. Moreover, we only require $n$ to be linear in the asymptotic polynomial growth rate of the graph; this confirms a conjecture of Levin and Linial, London, and Rabinovich "in the asymptotic sense." (The exact form of the conjecture was refuted by Krauthgamer and Lee.) All our results are proved for Borel graphs, which allows us to settle a number of problems in descriptive combinatorics. Roughly, we prove that graphs generated by free Borel actions of $\mathbb Z^n$ are universal for the class of Borel graphs of polynomial growth. This provides a general method for extending results about $\mathbb Z^n$-actions to all Borel graphs of polynomial growth. For example, an immediate consequence of our main result is that all Borel graphs of polynomial growth are hyperfinite, which answers a well-known question in the area. As another illustration, we show that Borel graphs of polynomial growth support a certain combinatorial structure called toast. An important technical tool in our arguments is the notion of padded decomposition from computer science, which is closely related to the concept of asymptotic dimension due to Gromov. Along the way we find an alternative, probabilistic proof of a theorem of Papasoglu that graphs of asymptotic polynomial growth rate $\rho<\infty$ have asymptotic dimension at most $\rho$ and establish the same bound in the Borel setting.