The Maximum k-Differential Coloring Problem

TL;DR

This paper characterizes which bipartite, planar, and outerplanar graphs can be colored with differential constraints, and proves NP-completeness for certain cases of the maximum k-differential coloring problem.

Contribution

It provides a complete characterization for specific graph classes and establishes NP-completeness results for broader cases of the maximum k-differential coloring problem.

Findings

Characterization of bipartite, planar, and outerplanar graphs admitting (2,kn)-differential coloring

NP-completeness of (3,2n)-differential coloring decision problem

NP-completeness of (loor{2n/3}, 2n)-differential coloring for planar graphs

Abstract

Given an $n$-vertex graph $G$ and two positive integers $d,k \in \mathbb{N}$, the ($d,kn$)-differential coloring problem asks for a coloring of the vertices of $G$ (if one exists) with distinct numbers from 1 to $kn$ (treated as \emph{colors}), such that the minimum difference between the two colors of any adjacent vertices is at least $d$. While it was known that the problem of determining whether a general graph is ($2,n$)-differential colorable is NP-complete, our main contribution is a complete characterization of bipartite, planar and outerplanar graphs that admit ($2,kn$)-differential colorings. For practical reasons, we consider also color ranges larger than $n$, i.e., $k > 1$. We show that it is NP-complete to determine whether a graph admits a ($3,2n$)-differential coloring. The same negative result holds for the ($\lfloor 2n/3 \rfloor, 2n$-differential coloring problem, even in the case where the input graph is planar.