**Conjecture**If is an orientation of a simple planar graph, then there is a partition of into so that the graph induced by is acyclic for .

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.

## Bibliography

* [N] V. Neumann-Lara (1985). Vertex colourings in digraphs. Some problems. Technical report, University of Waterloo.

* indicates original appearance(s) of problem.