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Construction 1 gives an induced vertex-Ramsey graph

PDF p. 2: Construction 1, monochromatic-hyperedge argument

Statement

Let H be a finite simple graph with at least one vertex, and let q≥3 and k≥1 be integers. For any choices defining the graph G=G(H,q,k) in Construction 1, every k-colouring of V(G) has a monochromatic hyperedge of the underlying hypergraph, and that hyperedge induces a copy of H in G. Consequently G∈Rindk(H). Such construction choices exist for every stated H,q,k.

Facts & Assumptions

[F1]

By Construction 1: graph G(H,q,k), the underlying finite ∣V(H)∣-uniform hypergraph H=(V,E) has Berge girth at least q and no weak k-colouring, and V(G)=V. Each hyperedge e is assigned a bijection fe:V(H)→e. The construction and High-girth hypergraphs for Construction 1 supply choices for every stated H,q,k, including the one-vertex case.

[F2]

By Unrestricted finite colourings and used palettes and Weak hypergraph colourings with singleton boundary, a k-colouring is any function into {1,…,k}, a subset is monochromatic when all its vertices receive the same colour, and such a function is a weak hypergraph colouring exactly when every hyperedge contains two vertices of different colours. Hypergraph and girth terminology is fixed by Hypergraph terminology used in Construction 1.

[F3]

By Copies on hyperedges are induced, each hyperedge e induces the transported piece of H, and fe is an induced embedding. An induced embedding preserves and reflects adjacency between distinct vertices, and its image induces a copy (Induced embeddings and induced copies of a graph). The graph and copy conventions are given in Graph and colouring conventions.

[F4]

By Induced vertex-Ramsey class, a nonempty finite simple graph belongs to Rindk(H) precisely when every function from its vertex set to {1,…,k} has a monochromatic induced copy of H.

Proof

Given: A graph H, integers q,k, and fixed construction choices as in the statement.

Goal: Prove the monochromatic-hyperedge assertion and the induced Ramsey membership, and account for the existence of the construction choices.

1.1givenF1F2choose

Let c:V(G)→{1,…,k} be arbitrary. Because V(G)=V, it is also a colouring of the hypergraph's vertex set. It cannot be a weak k-colouring, so some hyperedge e fails the condition of containing two differently coloured vertices. Thus all vertices of e have the same c-colour. Since ∣e∣=∣V(H)∣≥1, this is a nonempty monochromatic hyperedge.

2.1F1F3F4step 1.1∎

The induced subgraph of G on e is a copy of H under fe, so it is monochromatic by step 1.1. In particular G is nonempty. As c was arbitrary, the defining condition for G∈Rindk(H) holds. The choices used above exist for every stated H,q,k by F1. This proves all assertions.

Depends on

Used by · 3

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