Published online by Cambridge University Press: 18 June 2021
A (not necessarily proper) vertex colouring of a graph has clustering c if every monochromatic component has at most c vertices. We prove that planar graphs with maximum degree $\Delta$ are 3-colourable with clustering $O(\Delta^2)$. The previous best bound was $O(\Delta^{37})$. This result for planar graphs generalises to graphs that can be drawn on a surface of bounded Euler genus with a bounded number of crossings per edge. We then prove that graphs with maximum degree $\Delta$ that exclude a fixed minor are 3-colourable with clustering $O(\Delta^5)$. The best previous bound for this result was exponential in $\Delta$.
Research is supported by NSERC.
Partially supported by ANR Projects GATO (anr-16-CE40-0009-01) and GrR (anr-18-CE40-0032).
Research is supported by NSERC.
Research is partially supported by the National Science Centre of Poland grant 2015/17/D/ST1/00585.
Research is supported by the Australian Research Council.