Truncated degree AT-orientations of outerplanar graphs

TL;DR

This paper establishes new bounds on AT-orientations and paintability of 2-connected outerplanar graphs, improving previous results on degree-choosability and extending understanding of graph orientations.

Contribution

It proves that 2-connected outerplanar graphs (excluding odd cycles) are 5-truncated degree-AT and 4-truncated degree-AT, enhancing prior degree-choosability results.

Findings

2-connected outerplanar graphs (excluding odd cycles) are 5-truncated degree-AT.

2-connected bipartite outerplanar graphs are 4-truncated degree-AT.

These graphs are also 5-truncated degree paintable and 4-truncated degree paintable.

Abstract

An AT-orientation of a graph $G$ is an orientation $D$ of $G$ such that the number of even Eulerian sub-digraphs and the number of odd Eulerian sub-digraphs of $D$ are distinct. Given a mapping $f: V(G) \to \mathbb{N}$, we say $G$ is $f$-AT if $G$ has an AT-orientation $D$ with $ < f(v)$ for each vertex $v$. For a positive integer $k$, we say $G$ is $k$-truncated degree-AT if $G$ is $f$-AT for the mapping $f$ defined as $f(v) = \min #{k, d_G(v)#} $. This paper proves that 2-connected outerplanar graphs other than odd cycles are $5$-truncated degree-AT, and 2-connected bipartite outerplanar graphs are $4$-truncated degree-AT. As a consequence, 2-connected outerplanar graphs other than odd cycles are $5$-truncated degree paintable, and 2-connected bipartite outerplanar graphs are $4$-truncated degree paintable. This improves the result of Hutchinson in [On list-coloring outerplanar graphs], where it was proved that maximal 2-connected outerplanar graphs other than are 5-truncated degree-choosable, and 2-connected bipartite outerplanar graphs are 4-truncated degree-choosable.