A cyclic coloring of a planar embedding is a vertex coloring such that vertices incident with the same face have distinct colors. The minimum number of colors in a cyclic coloring of a graph is called the cyclic chromatic number of the graph, variously denoted χ_c (Plummer and Toft 1987, Zlámalová 2010) or χ^c .

chromatic number | planar embedding | Plummer-Toft graph