The Petersen graph occupies an important position in the development of several areas of modern graph theory because it often appears as a counter-example to important conjectures. In this account, the authors examine those areas, using the prominent role of the Petersen graph as a unifying feature. Topics covered include: vertex and edge colourability (including snarks), factors, flows, projective geometry, cages, hypohamiltonian graphs, and 'symmetry' properties such as distance transitivity. The final chapter contains a pot-pourri of other topics in which the Petersen graph has played its part. Undergraduate students will be able to profit from reading this book as the prerequisites are few; thus it could be used for a second course in graph theory. On the other hand, the authors have also included a number of unsolved problems as well as topics of recent study. Thus it will also be useful as a reference for graph theorists.

We investigate the values of t(n), the maximum number of edges in a graph with n vertices and not containing a four-cycle. Techniques for finding these are developed and the values of t(n) for all n up to 21 are obtained. All the corresponding extremal graphs are found.

The Heawood graph and $K_{3,3}$ have the property that all of their 2-factors are Hamilton circuits. We call such graphs 2-factor hamiltonian. We prove that if G is a k-regular bipartite 2-factor hamiltonian graph then either G is a circuit or k = 3. Furthermore, we construct an infinite family of cubic bipartite 2-factor hamiltonian graphs based on the Heawood graph and $K_{3,3}$ and conjecture that these are the only such graphs

For n = 1, 2, . . . , let 6, = K2+ K,,. We pose the problem of determining the Ramsey numbers r (&, B,) and demonstrate that in many cases critical colorings are available from known examples of strongly regular graphs.

No abstract