On $d$-dimensional nowhere-zero $r$-flows on a graph

Abstract

A $d$-dimensional nowhere-zero $r$-flow on a graph $G$, an $(r,d)$-NZF from now on, is a flow where the value on each edge is an element of ${\mathbb{R}}^d$ whose (Euclidean) norm lies in the interval $[1,r-1]$. Such a notion is a natural generalization of the well-known concept of circular nowhere-zero $r$-flow (i.e.\ $d=1$). For every bridgeless graph $G$, the $5$-flow Conjecture claims that ${\varphi }_1(G)\le 5$, while a conjecture by Jain suggests that ${\varphi }_d(G)=1$, for all $d\ge 3$. Here, we address the problem of finding a possible upper-bound also for the remaining case $d=2$. We show that, for all bridgeless graphs, ${\varphi }_2(G)\le 1+\sqrt{5}$ and that the oriented $5$-cycle double cover Conjecture implies ${\varphi }_2(G)\le {\tau }^2$, where $\tau$ is the Golden Ratio. Moreover, we propose a geometric method to describe an $(r,2)$-NZF of a cubic graph in a compact way, and we apply it in some instances. Our results and some computational evidence suggest that ${\tau }^2$ could be a promising upper bound for the parameter ${\varphi }_2(G)$ for an arbitrary bridgeless graph $G$. We leave that as a relevant open problem which represents an analogous of the $5$-flow Conjecture in the $2$-dimensional case (i.e. complex case).

14 pages, 4 figures