"center">
      Graph Representations and Data
    </h3>

    <p>
      Perhaps the simplest description of a graph is the <b>edge list</b>.
      This representation consists of a list of the pairs of nodes that
      are joined by an edge.  If (A,B) is given in the list, then
      the equivalent pair (B,A) is not listed.  For convenience, it is
      often the case that the pair is chosen in which the first node
      has a lower numeric index than the second.  Similarly, the order
      in which these e