Computing weighted Szeged and PI indices from quotient graphs

Abstract

The weighted Szeged index and the weighted vertex-PI index of a connected graph \(G\) are defined as \(wSz(G) = \sum_{e=uv \in E(G)} (deg (u) + deg (v))n_u(e)n_v(e)\) and \(wPI_v(G) = \sum_{e=uv \in E(G)} (deg(u) + deg(v))( n_u(e) + n_v(e))\), respectively, where \(n_u(e)\) denotes the number of vertices closer to \(u\) than to \(v\) and \(n_v(e)\) denotes the number of vertices closer to \(v\) than to \(u\). Moreover, the weighted edge-Szeged index and the weighted PI index are defined analogously. As the main result of this paper, we prove that if \(G\) is a connected graph, then all these indices can be computed in terms of the corresponding indices of weighted quotient graphs with respect to a partition of the edge set that is coarser than the \(\Theta^*\)-partition. If \(G\) is a benzenoid system or a phenylene, then it is possible to choose a partition of the edge set in such a way that the quotient graphs are trees. As a consequence, it is shown that for a benzenoid system the mentioned indices can be computed in sub-linear time with respect to the number of vertices. Moreover, closed formulas for linear phenylenes are also deduced. However, our main theorem is proved in a more general form and therefore, we present how it can be used to compute some other topological indices.