Study of inverse sum indeg index

Let mg(i,n) (1 ≤ i ≤ 3) denote to the class of all n-vertex molecular graphs with minimum degree i. the inverse sum indeg index of a graph is defined as isi = σ_uv∈e(g) dudv/(du + dv), where du denotes to the degree of vertex u. in this paper, we propose some extremal molecular graphs with the minimum and the maximum value of inverse sum indeg index in mg(i,n).