A note on counting flows in signed graphs

Abstract

Tutte initiated the study of nowhere-zero flows and proved the following fundamental theorem: For every graph $G$ there is a polynomial $f$ so that for every abelian group $Γ$ of order $n$, the number of nowhere-zero $Γ$-flows in $G$ is $f(n)$. For signed graphs (which have bidirected orientations), the situation is more subtle. For a finite group $Γ$, let $ε_2(Γ)$ be the largest integer $d$ so that $Γ$ has a subgroup isomorphic to $\mathbb{Z}_2^d$. We prove that for every signed graph $G$ and $d \ge 0$ there is a polynomial $f_d$ so that $f_d(n)$ is the number of nowhere-zero $Γ$-flows in $G$ for every abelian group $Γ$ with $ε_2(Γ) = d$ and $|Γ| = 2^d n$. Beck and Zaslavsky had previously established the special case of this result when $d=0$ (i.e., when $Γ$ has odd order).

7 pages