On difference of Zagreb indices

The classical first and second Zagreb indices of a graph G are defined as M1=∑vdv2 and M2=∑uvdudv, where dv is the degree of the vertex v of G. So far, the difference of M1 and M2 has not been studied. We show that this difference is closely related to the vertex-degree-based invariant RM2=∑uv(du−1)(dv−1), and determine a few basic properties of RM2.