## Introduction

A **graph** *G* consists of a set *V*, whose members are called the **vertices** of *G*, together with a set *E* of pairs of distinct vertices from *V*. These paris are called the **edges** of *G*. If *e = (v, w)* is an edge with vertices *v* and *w*, then *v* and *w* are said to **lie on** *e*, and *e* is said to be **incident** with *v* and *w*.