English

Disjoint list-colorings for planar graphs

Combinatorics 2023-12-29 v1

Abstract

One of Thomassen's classical results is that every planar graph of girth at least 55 is 3-choosable. One can wonder if for a planar graph GG of girth sufficiently large and a 33-list-assignment LL, one can do even better. Can one find 33 disjoint LL-colorings (a packing), or 22 disjoint LL-colorings, or a collection of LL-colorings that to every vertex assigns every color on average in one third of the cases (a fractional packing)? We prove that the packing is impossible, but two disjoint LL-colorings are guaranteed if the girth is at least 88, and a fractional packing exists when the girth is at least 6.6. For a graph GG, the least kk such that there are always kk disjoint proper list-colorings whenever we have lists all of size kk associated to the vertices is called the list packing number of GG. We lower the two-times-degeneracy upper bound for the list packing number of planar graphs of girth 3,43,4 or 55. As immediate corollaries, we improve bounds for ϵ\epsilon-flexibility of classes of planar graphs with a given girth. For instance, where previously Dvo\v{r}\'{a}k et al. proved that planar graphs of girth 66 are (weighted) ϵ\epsilon-flexibly 33-choosable for an extremely small value of ϵ\epsilon, we obtain the optimal value ϵ=13\epsilon=\frac{1}{3}. Finally, we completely determine and show interesting behavior on the packing numbers for HH-minor-free graphs for some small graphs H.H.

Keywords

Cite

@article{arxiv.2312.17233,
  title  = {Disjoint list-colorings for planar graphs},
  author = {Stijn Cambie and Wouter Cames van Batenburg and Xuding Zhu},
  journal= {arXiv preprint arXiv:2312.17233},
  year   = {2023}
}

Comments

36 pages, 8 figures

R2 v1 2026-06-28T14:04:02.133Z