Novel PI Indices of Hexagonal Chains

Problems don't hung on trees. Formulation of a good problem is often the most important part of a (theoretical) research. New problems usually arise when you try to solve old problems. Ivan Gutman, June 27, 1995.The Padmakar–Ivan (PI) index of hexagonal chains (i.e., the molecular graphs of unbranched catacondensed benzenoid hydrocarbons) is examined. The index PI is a graph invariant defined as the summation of the sums of edges of n _eu and n _ev over all the edges of connected graph G, where n _eu is the number of edges of G lying closer to u than to v and n _ev is the number of edges of G lying closer to v than to u. An efficient calculation of formula for PI for hexagonal chains are put forward.