A branch of topology that is concerned with representing the relations between pairs of entities: the entities are represented by nodes or points and their pair-wise connectivity by arcs or lines. Thus, the representation of a molecule by symbols for atoms joined by lines lends itself very naturally to graph theory. There is a substantial literature on the use of graph theory in chemistry, starting with Arthur Cayley’s estimation of the number of possible isomers of alkanes in 1874. Graph theory has also been used in discussions about chemical reactivity and the structures of solids.