PI polynomials of product graphs

The Padmakar–Ivan (PI) index of a graph G is defined as P I ( G ) = ∑ [ n e u ( e | G ) + n e v ( e | G ) ] , where n e u ( e | G ) is the number of edges of G lying closer to u than to v , n e v ( e | G ) is the number of edges of G lying closer to v than to u and summation goes over all edges of G . In this work, the PI polynomial for the Cartesian product graphs is computed.