Polynomials Counting Group Colorings in Graphs

Abstract: Jaeger et al. in 1992 introduced group coloring as the dual concept to group connectivity in graphs. Let $A$ be an additive Abelian group, $ f: E(G)\to A$ and $D$ an orientation of a graph $G$. A vertex coloring $c:V(G)\to A$ is an $(A, f)$-coloring if $c(v)-c(u)\ne f(e)$ for each oriented edge $e=uv$ from $u$ to $v$ with respect to $D$. Kochol recently introduced the assigning polynomial counting nowhere-zero chains in graphs and regular matroids. Motivated by Kochol's work, we define the $\alpha$-compatible graph and the cycle-assigning polynomial $P(G, \alpha; k)$ at $k$ in terms of $\alpha$-compatible spanning subgraphs, where $\alpha$ is an assigning of $G$ from its cycles to ${0,1}$. We prove that $P(G,\alpha;k)$ evaluates the number of $(A,f)$-colorings of $G$ for any Abelian group $A$ of order $k$ and $f:E(G)\to A$ such that the assigning $\alpha_{D,f}$ given by $f$ equals $\alpha$. Based on Kochol's work, we also show that $k^{-c(G)}P(G,\alpha;k)$ is a polynomial enumerating $(A,f)$-tensions and counting specific nowhere-zero chains. By extending the concept of broken cycle to broken compatible cycles, associated with any admissible assigning $\alpha$ of $G$, we demonstrate that the absolute value of the coefficient of $k^{|V(G)|-i}$ in $P(G,\alpha;k)$ equals the number of $\alpha$-compatible spanning subgraphs that have $i$ edges and contain no broken $\alpha$-compatible cycles. This generalizes the Whitney's celebrated Broken Cycle Theorem. Based on the combinatorial explanation, we establish a unified order-preserving relation from admissible assignings to cycle-assigning polynomials. Finally, we show that for any admissible assigning $\alpha$ of $G$ with $\alpha(e)=1$ for every loop $e$ of $G$, the coefficients of the cycle-assigning polynomial $P(G,\alpha;k)$ are nonzero and alternate in sign.