English

Many flows in the group connectivity setting

Combinatorics 2020-05-21 v1

Abstract

Two well-known results in the world of nowhere-zero flows are Jaeger's 4-flow theorem asserting that every 4-edge-connected graph has a nowhere-zero Z2×Z2\mathbb{Z}_2 \times \mathbb{Z}_2-flow and Seymour's 6-flow theorem asserting that every 2-edge-connected graph has a nowhere-zero Z6\mathbb{Z}_6-flow. Dvo\v{r}\'ak and the last two authors of this paper extended these results by proving the existence of exponentially many nowhere-zero flows under the same assumptions. We revisit this setting and provide extensions and simpler proofs of these results. The concept of a nowhere-zero flow was extended in a significant paper of Jaeger, Linial, Payan, and Tarsi to a choosability-type setting. For a fixed abelian group Γ\Gamma, an oriented graph G=(V,E)G = (V,E) is called Γ\Gamma-connected if for every function f:EΓf : E \rightarrow \Gamma there is a flow ϕ:EΓ\phi : E \rightarrow \Gamma with ϕ(e)f(e)\phi(e) \neq f(e) for every eEe \in E (note that taking f=0f = 0 forces ϕ\phi to be nowhere-zero). Jaeger et al. proved that every oriented 3-edge-connected graph is Γ\Gamma-connected whenever Γ6|\Gamma| \ge 6. We prove that there are exponentially many solutions whenever Γ8|\Gamma| \ge 8. For the group Z6\mathbb{Z}_6 we prove that for every oriented 3-edge-connected G=(V,E)G = (V,E) with =EV11\ell = |E| - |V| \ge 11 and every f:EZ6f: E \rightarrow \mathbb{Z}_6, there are at least 2/log2^{ \sqrt{\ell} / \log \ell} flows ϕ\phi with ϕ(e)f(e)\phi(e) \neq f(e) for every eEe \in E.

Keywords

Cite

@article{arxiv.2005.09767,
  title  = {Many flows in the group connectivity setting},
  author = {Matt DeVos and Rikke Langhede and Bojan Mohar and Robert Šámal},
  journal= {arXiv preprint arXiv:2005.09767},
  year   = {2020}
}

Comments

19 pages

R2 v1 2026-06-23T15:40:28.245Z