Big Ramsey degrees of 3-uniform hypergraphs

Given a countably infinite hypergraph $\mathcal R$ and a finite hypergraph $\mathcal A$, the big Ramsey degree of $\mathcal A$ in $\mathcal R$ is the least number $L$ such that, for every finite $k$ and every $k$-colouring of the embeddings of $\mathcal A$ to $\mathcal R$, there exists an embedding $f$ from $\mathcal R$ to $\mathcal R$ such that all the embeddings of $\mathcal A$ to the image $f(\mathcal R)$ have at most $L$ different colours. We describe the big Ramsey degrees of the random countably infinite 3-uniform hypergraph, thereby solving a question of Sauer. We also give a new presentation of the results of Devlin and Sauer on, respectively, big Ramsey degrees of the order of the rationals and the countably infinite random graph. Our techniques generalise (in a natural way) to relational structures and give new examples of Ramsey structures (a concept recently introduced by Zucker with applications to topological dynamics).Comment: 8 pages, 3 figures, extended abstract for Eurocomb 201

Big Ramsey degrees and infinite languages

This paper investigates big Ramsey degrees of unrestricted relational structures in (possibly) infinite languages. While significant progress has been made in studying big Ramsey degrees, many classes of structures with finite small Ramsey degrees still lack an understanding of their big Ramsey degrees. We show that if there are only finitely many relations of every arity greater than one, then unrestricted relational structures have finite big Ramsey degrees, and give some evidence that this is tight. This is the first time that finiteness of big Ramsey degrees has been established for an infinite-language random structure. Our results represent an important step towards a better understanding of big Ramsey degrees for structures with relations of arity greater than two.Comment: 21 pages. An updated version strengthening the statement of the positive results and fixing a mistake in the earlier version of the negative result which now needs an extra assumptio