Partitioning planar digraphs
This is a type of coloring digraphs introduced by V. Neumann-Lara. More generally, if is a digraph, we wish to partition the vertex set of into as few parts as possible so that each induces an acyclic subgraph.
* [N] V. Neumann-Lara (1985). Vertex colourings in digraphs. Some problems. Technical report, University of Waterloo.
* indicates original appearance(s) of problem.