Trees, Quadratic Line Graphs and the Wiener Index

The Wiener index is a topological index defined as the sum of distances between all pairs of vertices in a tree. It was introduced as a structural descriptor for molecular graphs of alkanes, which are trees with vertex degrees of four at the most (chemical trees). The line graph L(G) of a graph G has the vertex set V(L(G)) = E(G) and two distinct vertices of L(G) are adjacent if the corresponding edges of G have a common endvertex. It is known that the Wiener indices of a tree and of its line graph are always distinct. An infinite two-parameter family of growing chemical trees T with the property W(T) = W(L(L(T))) has been constructed. Key words topological index Wiener index tree line graph * Authors to whom correspondence should be addressed. (E-mail: