The PI index of product graphs

The Padmakar–Ivan index of a graph GG is the sum over all edges uvuv of GG of number of edges which are not equidistant from uu and vv. In this work, an exact expression for the PI index of the Cartesian product of bipartite graphs is computed. Using this formula, the PI indices of C4C4 nanotubes and nanotori are computed.