A necessary and sufficient condition for the existence of $\{p,p+1,q-1,q\}$-orientations in simple graphs

TL;DR

This paper establishes a precise condition for the existence of certain orientations in simple graphs, linking specific out-degree sets to degree-constrained orientations, and refines related bipartite graph factor results.

Contribution

It provides a necessary and sufficient condition for orientations with out-degrees in a specific set, improving understanding of degree constraints in simple graphs.

Findings

Characterizes orientations with out-degrees in \\{p,p+1,q-1,q\\}

Shows equivalence to orientations with out-degrees in \\{p,...,q\\}

Refines bipartite graph degree-constrained factor results

Abstract

Let $G$ be a simple graph and let $p$ and $q$ be two integer-valued functions on $V(G)$ with $p< q$ in which for each $v\in V(G)$, $q(v) \ge \frac{1}{2}d_G(v)$ and $p(v) \ge \frac{1}{2} q(v)-2$. In this note, we show that $G$ has an orientation such that for each vertex $v$, $d^+_G(v)\in\{p(v),p(v)+1,q(v)-1,q(v)\}$ if and only if it has an orientation such that for each vertex $v$, $p(v) \le d^+_G(v)\le q(v)$ where $d^+_G(v)$ denotes the out-degree of $v$ in $G$. From this result, we refine a result due to Addario-Berry, Dalal, and Reed (2008) in bipartite simple graphs on the existence of degree constrained factors.