Jaeger conjecture and Jaeger-Zhang conjecture

Abstract

We prove that every signed planar graph $\widehat G$ of negative girth at least $\ell\geq 3$ satisfies $χ_c(\widehat G)<\frac{2\ell}{\ell-2}$. As a consequence, for every integer $k\geq 1$, every planar graph $G$ of odd girth at least $4k+3$ satisfies $χ_c(G)<\frac{2k+1}{k}$ and hence admits a homomorphism to $C_{2k+1}$. Our proofs build on recent results of Jafari on circular flows. In particular, Jafari proved that every $(4k+2)$-edge-connected graph has circular flow number strictly less than $2+\frac{1}{k}$. We revisit his theorem, providing an alternative proof that emphasizes the underlying graph-theoretic ideas. We also extend the result to signed graphs and weaken the connectivity assumption by imposing a lower bound only on the size of negative cuts. The key step is a small-correction lemma associated with a distribution on spanning trees. A splitting argument first gives the required correction in an even-degree graph; doubling the edges removes the degree-parity assumption. Plane duality then gives the coloring theorem. We discuss tightness of bounds in the end.