Published online by Cambridge University Press: 05 May 2015
We discuss the colouring theory of finite set systems. This is not merely an extension of results from collections of 2-element sets (graphs) to larger sets. The wider structure (hypergraphs) offers many interesting new kinds of problems, which either have no analogues in graph theory or become trivial when we restrict them to graphs.
Introduction
In this introductory section we give the most important definitions required to study hypergraph colouring, and briefly survey the half-century history of this topic. For more details on the material of Sections 1 and 2 we refer to Berge [8], Zykov [76] and Duchet [27].
Let V = {v1, v2, …, vn} be a finite set of elements called vertices, and let ℇ = {E1, E2, …, Em} be a family of subsets of V called edges or hyperedges. The pair ℌ = (V, ℇ) is called a hypergraph with vertex-set V = V(ℌ) and edge-set ℇ = ℇ(ℌ). The hypergraph ℌ = (V, ℇ) is sometimes called a set system. If each edge of a hypergraph contains precisely two vertices, then it is a graph. As in graph theory, the number |V| = n is called the order of the hypergraph. Edges with fewer than two elements are usually allowed, but will be disregarded here. Thus, throughout this chapter we assume that each edge E ∈ ℇ contains at least two vertices, unless stated explicitly otherwise. Edges that coincide are called multiple edges.
In a hypergraph, two vertices are said to be adjacent if there is an edge containing both of these vertices. The adjacent vertices are sometimes called neighbours of each other, and the set of neighbours of a given vertex v is called the (open) neighbourhood N(v) of v. If v ∈ E, then the vertex v and the edge E are incident with each other. For an edge E, the number |E| is called the size or cardinality of E.
To save this book to your Kindle, first ensure [email protected] is added to your Approved Personal Document E-mail List under your Personal Document Settings on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part of your Kindle email address below. Find out more about saving to your Kindle.
Note you can select to save to either the @free.kindle.com or @kindle.com variations. ‘@free.kindle.com’ emails are free but can only be saved to your device when it is connected to wi-fi. ‘@kindle.com’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.
Find out more about the Kindle Personal Document Service.
To save content items to your account, please confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account. Find out more about saving content to Dropbox.
To save content items to your account, please confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account. Find out more about saving content to Google Drive.