Definitions for "graph G"
a finite set, V(G) of objects, called "vertices", together with a set, E(G), of unordered pairs of distict vertices
a k-leaf power, fixed k if there is a tree T whose leaves correspond to the vertices of G in such a way that two vertices are adjacent in G precisely when their distance in T is at most k
a k -tree if it is K k or is obtained from a k -tree by adding a new vertex adjacent to a k -clique
a Helly circle graph if G is a circle graph and there exists a model of G by chords such that every three pairwise intersecting chords intersect at the same point
an e-circle graph if and only if it is a circle graph
an e-circle graph if we can find a model of straight lines on the cartesian plane according to the following procedure
a divisor graph if there exists a set S of positive integers such that G is isomorphic to G(S)
Keywords:  iff, tolerance, path
a path tolerance graph (PT-graph) iff G is a PT(t)-graph for some tolerance t
Keywords:  interval, www
an interval www
Keywords:  extremal
a extremal graph if and only