On the PI index of some nanotubes

The PI index is a graph invariant defined as the summation of the sums of edges of neu and nev over all the edges of connected graph G, where neu is the number of edges of G lying closer to u than to v and nev is the number of edges of G lying closer to v than to u. The index is very simple to calculate and has disseminating power similar to that of the Wiener and the Szeged indices. The comprehensive studies show that the PI index correlates highly with W and Sz as well as with physicochemical properties and biological activities of a large number of diversified and complex compounds. In this paper we prove an algorithm which is very simple for computing PI index of nanotubes. Using this algorithm the PI index of a polyhex zig-zag nanotube is computed