Note on semi-proper orientations of outerplanar graphs

TL;DR

This paper investigates semi-proper orientations of outerplanar graphs, establishing tight bounds on their semi-proper orientation number, which is a weighted generalization of proper orientation, and extends previous results on cactus graphs.

Contribution

It proves tight bounds for the semi-proper orientation number of cactus and outerplanar graphs, advancing understanding of graph orientations with weighted constraints.

Findings

For every cactus graph, the semi-proper orientation number is at most 3.

The bound of 3 for cactus graphs is tight.

For every outerplanar graph, the semi-proper orientation number is at most 4.

Abstract

A semi-proper orientation of a given graph $G$, denoted by $(D,w)$, is an orientation $D$ with a weight function $w: A(D)\rightarrow \mathbb{Z}_+$, such that the in-weight of any adjacent vertices are distinct, where the in-weight of $v$ in $D$, denoted by $w^-_D(v)$, is the sum of the weights of arcs towards $v$. The semi-proper orientation number of a graph $G$, denoted by $\overrightarrow{\chi}_s(G)$, is the minimum of maximum in-weight of $v$ in $D$ over all semi-proper orientation $(D,w)$ of $G$. This parameter was first introduced by Dehghan (2019). When the weights of all edges eqaul to one, this parameter is equal to the proper orientation number of $G$. The optimal semi-proper orientation is a semi-proper orientation $(D,w)$ such that $\max_{v\in V(G)}w_D^-(v)=\overrightarrow{\chi}_s(G)$. Ara\'ujo et al. (2016) showed that $\overrightarrow{\chi}(G)\le 7$ for every cactus $G$ and the bound is tight. We prove that for every cactus $G$, $\overrightarrow{\chi}_s(G) \le 3$ and the bound is tight. Ara\'{u}jo et al. (2015) asked whether there is a constant $c$ such that $\overrightarrow{\chi}(G)\le c$ for all outerplanar graphs $G.$ While this problem remains open, we consider it in the weighted case. We prove that for every outerplanar graph $G,$ $\overrightarrow{\chi}_s(G)\le 4$ and the bound is tight.