This is not a traditional work on topological graph theory. No current
graph or voltage graph adorns its pages. Its readers will not compute
the genus (orientable or non-orientable) of a single non-planar graph.
Their muscles will not flex under the strain of lifting walks from base
graphs to derived graphs. What is it, then? It is an attempt to place
topological graph theory on a purely combinatorial yet rigorous footing.
The vehicle chosen for this purpose is the con- cept of a 3-graph, which
is a combinatorial generalisation of an imbedding. These properly
edge-coloured cubic graphs are used to classify surfaces, to generalise
the Jordan curve theorem, and to prove Mac Lane's characterisation of
planar graphs. Thus they playa central role in this book, but it is not
being suggested that they are necessarily the most effective tool in
areas of topological graph theory not dealt with in this volume.
Fruitful though 3-graphs have been for our investigations, other jewels
must be examined with a different lens. The sole requirement for
understanding the logical development in this book is some elementary
knowledge of vector spaces over the field Z2 of residue classes modulo
2. Groups are occasionally mentioned, but no expertise in group theory
is required. The treatment will be appreciated best, however, by readers
acquainted with topology. A modicum of topology is required in order to
comprehend much of the motivation we supply for some of the concepts
introduced.