PI indices of tori Tp,q[C4, C8] covered by C4 and C8

The Padmakar-Ivan (PI) index of a graph G = (VE) is defined as PI(G) = Sigma(e is an element of E)(n(u)(e)+ n(v)(e)), where e = uv, n(u)(e) is the number of edges of G lying closer to u than to v and n(v)(e) is the number of edges of G lying closer to v than to u. In this paper, a formula for calculating the PI index of a torus T-p,T-q[C-4, C-8] is given.