Abstract
Considering all partitions of the edges of a graph G to k parts, the the k-chromatic number of G is is the maximum of the sum of the chromatic numbers of the parts. We derive a Heawood-type formula for the k-chromatic number of graphs embedded in a fixed surface, improving the previously known upper bounds.
In infinitely many cases, the new upper bound coincides with the lower bound obtained from embedding disjoint cliques in the surface. In the proof of this result, we derive a variant of Euler's Formula for union of several graphs that might be interesting independently.