Connected equitably $Δ$-colorable realizations with $k$-factors
James M. Shook
TL;DR
This work advances the Chen–Lih–Wu equitable coloring program for connected graphs by constructing realizations with a $k$-factor that are equitably colorable with $Δ(G)$ colors and are connected (often with strong edge-connectivity). It develops generalized edge-exchanges that preserve $k$-factors, and adapts edge-connectivity techniques to ensure the leftover subgraph remains compatible with equitable colorings. Three main theorems—MaxDegreeEquitable, ExistDegreeEquitable, and a degree-sequence corollary—together establish that, for all $k$, there exists a connected equitably $Δ(G)$-colorable realization with a $k$-factor, providing substantial evidence toward the Chen–Lih–Wu conjecture. The results combine packing/embedding methods with degree-sequence arguments (via the strong index $m(π)$) to connect equitable colorability with the presence of regular factors, yielding both theoretical insight and potential tools for graph packing problems.
Abstract
A graph $G$ is said to be equitably $c$-colorable if its vertices can be partitioned into $c$ independent sets that pairwise differ in size by at most one. Chen, Lih, and Wu conjectured that every connected graph $G$ with maximum degree $Δ(G)\geq 2$ has an equitable coloring with $Δ(G)$ colors, except when $G$ is complete, an odd cycle, or a balanced bipartite graph with odd sized partitions. Suppose $G$ is a connected graph with a $k$-factor (a regular spanning subgraph) $F$ such that $G$ is not complete, a $1$-factor, nor an odd cycle. When $k\geq 1$ we demonstrate that there is a connected $(k-1)$ edge-connected equitably $Δ(G)$-colorable graph $H$ with a $k$-factor $F'$ such that $G-E(F)=H-E(F')$. If we drop the requirement that $G-E(F)=H-E(F')$, then we can say more. Considering the non-increasing degree sequence $π=(d_{1},\ldots, d_{n})$ of $G$ where $d_{i}=deg_{G}(v_{i})$ for all vertices $\{v_{1},\ldots,v_{n}\}$ of $G$, we call $m(π)=\max\{i|d_{i}\geq i\}$ the strong index of $π$. For $k\geq 0$, we can show that for every $$c\geq \max_{l\leq m(π)}\bigg\{\bigg\lfloor\frac{d_{l}+l}{2}\bigg\rfloor\bigg\}+1$$ we can find a connected $(k-1)$ edge-connected equitably $c$-colorable realization $H$ of $π$ that has a $k$-factor. In a third theorem we show that if $d_{d_{1}-d_{n}+1}\geq d_{1}-d_{n}+k-1$, then some realization of $π$ has a $k$-factor. Together, these three theorems allow us to prove that for all $k$, there is a connected equitably $Δ(G)$-colorable realization $H$ of $π$ with a $k$-factor. Thus, giving support to the validity of the Chen-Lih-Wu Conjecture.