Computing the PI index of some chemical graphs related to nanostructures

The Padmakar-Ivan (PI) index of a graph G is defined as PI(G) = ∑[n_eu(e|G) + n_ev(e|G)], where n_eu(e|G) is the number of edges of G lying closer to u than to v, n_ev(e|G) is the number of edges of G lying closer to v than to u and summation goes over all edges of G. The PI Index is a Szeged-like topological index developed very recently. In this paper we report on new results about computing PI index of nanotubes.