It is often helpful and visually appealing, to depict some situation which is of interest by a graphical figure consisting of points (vertices)—representing entities—and lines (links) joining certain pairs of these vertices and representing relationships between them. Such figures are known by the general name graphs and this book is devoted to their study. Graphs are met with everywhere under different names: "structures" in civil engineering, "networks" in electrical engineering, "sociograms", "communication structures" and "organizational structures" in sociology and economics, "molecular structure" in chemistry, "road maps", gas or electricity "distribution networks" and so on.