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 $\tilde{G}$ is obtained from $G$ by orienting some of its edges. Let $H(\tilde{G})$ denote the Hermitian adjacency matrix of $\tilde{G}$ and $A(G)$ be the adjacency matrix of $G$. The $H$-rank (resp. rank) of $\tilde{G}$ (resp. $G$), written as $rk(\tilde{G})$ (resp. $r(G)$), is the rank of $H(\tilde{G})$ (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 $\tilde{G}$ and the rank of $G$. We first show that $-2d(G)⩽rk(\tilde{G})-r(G)⩽2d(G)$ for every mixed graph $\tilde{G}$. 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.