Proper orientation number of triangle-free bridgeless outerplanar graphs

Jiangdong Ai, Stefanie Gerke, Gregory Gutin, Yongtang Shi, Zhenyu Taoqiu

Research output: Contribution to journalArticlepeer-review

44 Downloads (Pure)


An orientation of a graph G is a digraph obtained from G by replacing each edge by exactly one of two possible arcs with the same endpoints. We call an orientation proper if neighbouring vertices have different in-degrees. The proper orientation number of a graph G, denoted by \chi(G), is the minimum maximum in-degree of a proper orientation of G. Araujo et al. (Theor. Comput. Sci. 639 (2016) 14--25) asked whether there is a constant c such that \chi(G)< c for every outerplanar graph $G$ and showed that \chi}(G) <= 7$ for every cactus G. We prove that \chi}(G) <=3 if G is a triangle-free 2-connected outerplanar graph and $\chi (G) <= 4$ if G is a triangle-free bridgeless outerplanar graph.
Original languageEnglish
Pages (from-to)1-11
Number of pages11
JournalJournal of Graph Theory
Early online date20 Apr 2020
Publication statusE-pub ahead of print - 20 Apr 2020

Cite this