This chapter gives a gentle yet concise introduction to most of the terminology
used later in the book. Fortunately, much of standard graph
theoretic terminology is so intuitive that it is easy to remember; the few
terms better understood in their proper setting will be introduced later,
when their time has come.
Section 1.1 o®ers a brief but self-contained summary of the most
basic de¯nitions in graph theory, those centred round the notion of a
graph. Most readers will have met these de¯nitions before, or will have
them explained to them as they begin to read this book. For this reason,
Section 1.1 does not dwell on these de¯nitions more than clarity requires:
its main purpose is to collect the most basic terms in one place, for easy
reference later.
From Section 1.2 onwards, all new de¯nitions will be brought to life
almost immediately by a number of simple yet fundamental propositions.
Often, these will relate the newly de¯ned terms to one another: the
question of how the value of one invariant in°uences that of another
underlies much of graph theory, and it will be good to become familiar
with this line of thinking early.
By N we denote the set of natural numbers, including zero. The set
Z=nZ of integers modulo n is denoted by Zn; its elements are written as Zn
i := i+nZ. For a real number x we denote by bxc the greatest integer
6 x, and by dxe the least integer > x. Logarithms written as `log' are bxc; dxe
taken at base 2; the natural logarithm will be denoted by `ln'. A set log; ln
A = fA1; : : : ; Ak g of disjoint subsets of a set A is a partition of A if partition
A =
Sk
i=1 Ai and Ai 6= ; for every i. Another partition fA0
1; : : : ; A0
`
g of
A re¯nes the partition A if each A0
i is contained in some Aj. By [A]k we [A]k
denote the set of all k-element subsets of A. Sets with k elements will
be called k-sets; subsets with k elements are k-subsets.