CACTI WITH EXTREMAL PI INDEX

The vertex PI index PI(G) = Σ xy2E(G)[nxy(x) + nxy(y)] is a distance-based molecular structure descriptor, where nxy(x) denotes the number of vertices which are closer to the vertex x than to the vertex y and which has been the considerable research in computational chemistry dating back to Harold Wiener in 1947. A connected graph is a cactus if any two of its cycles have at most one common vertex. In this paper, we completely determine the extremal graphs with the greatest and smallest vertex PI indices mong all cacti with a xed number of vertices. As a consequence, we obtain the sharp bounds with corresponding extremal cacti and extend a known result.