# Borel sets of Rado graphs and Ramsey's Theorem

## TL;DR

This paper extends the classical Galvin-Prikry Ramsey theorem to the Rado graph, establishing a new infinite-dimensional Ramsey property for this homogeneous structure, which is significant for connections to topological dynamics.

## Contribution

It proves an analogue of the Galvin-Prikry theorem for the Rado graph, advancing the understanding of Ramsey properties in homogeneous structures beyond the Baire space.

## Key findings

- Established a Ramsey theorem for the Rado graph.
- Developed new fusion sequence techniques for the proof.
- Extended the framework of infinite dimensional Ramsey theory.

## Abstract

The well-known Galvin-Prikry Theorem states that Borel subsets of the Baire space are Ramsey: Given any Borel subset $\mathcal{X}\subseteq [\omega]^{\omega}$, where $[\omega]^{\omega}$ is endowed with the metric topology, each infinite subset $X\subseteq \omega$ contains an infinite subset $Y\subseteq X$ such that $[Y]^{\omega}$ is either contained in $\mathcal{X}$ or disjoint from $\mathcal{X}$. Kechris, Pestov, and Todorcevic point out in their seminal 2005 paper the dearth of similar results for homogeneous structures. Such results are a necessary step to the larger goal of finding a correspondence between structures with infinite dimensional Ramsey properties and topological dynamics, extending their correspondence between the Ramsey property and extreme amenability. In this article, we prove an analogue of the Galvin-Prikry theorem for the Rado graph. Any such infinite dimensional Ramsey theorem is subject to constraints following from the 2006 work of Laflamme, Sauer, and Vuksanovic. The proof uses techniques developed for the author's work on the Ramsey theory of the Henson graphs as well as some new methods for fusion sequences, used to bypass the lack of a certain amalgamation property enjoyed by the Baire space.