On b-greedy colourings and z-colourings

Abstract

A b-greedy colouring is a colouring which is both a b-colouring and a greedy colouring. A z-colouring is a b-greedy colouring such that a b-vertex of the largest colour is adjacent to a b-vertex of every other colour. The b-Grundy number (resp. z-number) of a graph is the maximum number of colours in a b-greedy colouring (resp. z-colouring) of it. In this paper, we study those two parameters. We show that similarly to the z-number, the b-Grundy number is not monotone and can be arbitrarily smaller than the minimum of the Grundy number and the b-chromatic number. We also describe a polynomial-time algorithm that decides whether a given $k$-regular graph has b-Grundy number (resp. z-number) equal to $k+1$. We also prove that every cubic graph with no induced 4-cycle has b-Grundy number and z-number exactly 4.