Juan Liu, Hong Yang, Xindong Zhang, Hongjian Lai
The characterization of line graphs helps to simplify the structure of (di) graphs, explore graph isomorphism, study matching and covering problems in graphs, analyze connectivity, and compute graph properties. The characterization of line graphs is significant for graph theory and its applications in various fields. As two special cases of mixed graphs, graphs and digraphs have many existing results on the characterization of their line (di)graphs. This paper mainly focuses on the characterization of line graphs for mixed graphs. The main results obtained in this paper are as follows: Let $H$ be a connected mixed graph with at least four vertices. $H$ is the line graph of a triangle-free mixed graph if and only if the edges of $H$ can be divided into a $T_{F} $-set or $H^{U} $ is $\{K_{1,3}, K_{1,1,2}\} $-free and $H$ is $\mathcal{Q}$-free; Furthermore, for bipartite mixed graphs and tree mixed graphs, the characterization of their line graphs is obtained.