This is the first book to focus on the topological index, the Harary
index, of a graph, including its mathematical properties, chemical
applications and some related and attractive open problems. This book is
dedicated to Professor Frank Harary (1921--2005), the grandmaster of
graph theory and its applications. It has be written by experts in the
field of graph theory and its applications. For a connected graph G, as
an important distance-based topological index, the Harary index H(G) is
defined as the sum of the reciprocals of the distance between any two
unordered vertices of the graph G. In this book, the authors report on
the newest results on the Harary index of a graph. These results mainly
concern external graphs with respect to the Harary index; the relations
to other topological indices; its properties and applications to pure
graph theory and chemical graph theory; and two significant variants,
i.e., additively and multiplicatively weighted Harary indices. In the
last chapter, we present a number of open problems related to the Harary
index. As such, the book will not only be of interest to graph
researchers, but to mathematical chemists as well.