Abstract
In a recent article, Nadeem and Siddique used Chebyshev's sum inequality to establish the Zagreb indices inequality \(M_1/n\le M_2/m\) for undirected graphs in the case where the degree sequence \((d_i)\) and the degree-sum sequence \((S_i)\) are similarly ordered.
We show that this is actually not a completely new result and we discuss several related results that also cover similar inequalities for directed graphs, as well as sum-symmetric matrices and Eulerian directed graphs.
