The gap-cycle conjecture for 2-coloring simple planar digraphs
Let be a simple planar digraph. Say that has no cycles of lengths when it contains no directed cycles whose lengths are among those integers. Gap-cycle conjecture. There exists such that every simple planar digraph without cycles of length is 2-colorable. This is presented as a relaxation of the Neumann-Lara–Škrekovski conjecture; the source says that even the original conjecture remains out of reach, so this question is open.
References
Primary source
Ararat Harutyunyan and Bojan Mohar, “Planar digraphs of digirth five are 2-colorable”, arXiv:1401.2213 (2014).
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