Relation between the H-rank of a mixed graph and the rank of its underlying graph

Abstract

Given a simple graph $G=(V_G, E_G)$ with vertex set $V_G$ and edge set $E_G$, the mixed graph $\widetilde{G}$ is obtained from $G$ by orienting some of its edges. Let $H(\widetilde{G})$ denote the Hermitian adjacency matrix of $\widetilde{G}$ and $A(G)$ be the adjacency matrix of $G$. The $H$-rank (resp. rank) of $\widetilde{G}$ (resp. $G$), written as $rk(\widetilde{G})$ (resp. $r(G)$), is the rank of $H(\widetilde{G})$ (resp. $A(G)$). Denote by $d(G)$ the dimension of cycle spaces of $G$, that is $d(G) = |E_G|-|V_G|+\omega(G)$, where $\omega(G),$ denotes the number of connected components of $G$. In this paper, we concentrate on the relation between the $H$-rank of $\widetilde{G}$ and the rank of $G$. We first show that $-2d(G)\leqslant rk(\widetilde{G})-r(G)\leqslant 2d(G)$ for every mixed graph $\widetilde{G}$. Then we characterize all the mixed graphs that attain the above lower (resp. upper) bound. By these obtained results in the current paper, all the main results obtained in \cite{004,1} may be deduced consequently.