A combinatorial model for the Menger curve

TL;DR

This paper models the universal Menger curve using a projective Fra"issé limit of finite connected graphs, establishing its homogeneity and universality properties through combinatorial and topological methods.

Contribution

It introduces a combinatorial Fra"issé limit approach to the Menger curve, proving new homogeneity theorems and extending the projective Fra"issé method to higher dimensions.

Findings

Proves the approximate projective homogeneity theorem for the Menger curve.

Recovers Anderson's finite homogeneity theorem.

First use of the projective Fra"issé method to prove an injective homogeneity theorem.

Abstract

We represent the universal Menger curve as the topological realization $|\mathbb{M}|$ of the projective Fra\"iss\'e limit ${\mathbb M}$ of the class of all finite connected graphs. We show that $\mathbb{M}$ satisfies combinatorial analogues of the Mayer-Oversteegen-Tymchatyn homogeneity theorem and the Anderson-Wilson projective universality theorem. Our arguments involve only $0$-dimensional topology and constructions on finite graphs. Using the topological realization $\mathbb{M}\mapsto|\mathbb{M}|$, we transfer some of these properties to the Menger curve: we prove the approximate projective homogeneity theorem, recover Anderson's finite homogeneity theorem, and prove a variant of Anderson-Wilson's theorem. The finite homogeneity theorem is the first instance of an "injective" homogeneity theorem being proved using the projective Fra\"iss\'e method. We indicate how our approach to the Menger curve may extend to higher dimensions.