Rank-Average Degree Bound for Graph Energy

Abstract

We prove that the energy ${\mathcal E}(G)$ of any simple graph $G$ of order $n\ge5$ satisfies \[ {\mathcal E}\ge r(G)+\bar d(G)-1, \] where $r(G)$ and $\bar d(G)$ denote, respectively, the rank of the adjacency matrix and the average degree of $G$. We also characterize all extremal graphs. As consequences, our result settles five previously conjectured lower bounds for the energy of nonsingular graphs in their stated ranges, namely \[ \begin{gathered} {\mathcal E}(G)\ge n-1+\bar d(G),\qquad {\mathcal E}(G)\ge\Delta(G)+\delta(G),\qquad {\mathcal E}(G)\ge2\sqrt{\bar d(G) (n-1)},\qquad {\mathcal E}(G)\ge\frac{M_1(G)}{m},\qquad {\mathcal E}(G)\ge\frac{M_1(G)}{2m}+\frac{2m}{n}, \end{gathered} \] where $m$ is the number of edges, $\Delta(G)$ and $\delta(G)$ are the maximum and minimum degrees, and the first Zagreb index $ M_1(G)$ is the sum of degree squares.