Weak diameter coloring of graphs on surfaces

Consider a graph G drawn on a fixed surface, and assign to each vertex a list of colors of size at least two if G is triangle-free and at least three otherwise. We prove that we can give each vertex a color from its list so that each monochromatic connected subgraph has bounded weak diameter (i.e., diameter measured in the metric of the whole graph G , not just the subgraph). In case that G has bounded maximum degree, this implies that each connected monochromatic subgraph has bounded size. This solves a problem of Esperet and Joret for planar triangle-free graphs, and extends known results in the general case to the list setting, answering a question of Wood. (c) 2023 Elsevier Ltd. All rights reserved.