Converse-invariance conjecture for regular Seymour-tight orientations

Let OO be an orientation, and write Ni−(O,v)N^-_i(O,v) and Ni+(O,v)N^+_i(O,v) for the in- and out-neighbourhoods of distance ii from vv. Call OO Seymour-tight when ∣N2+(O,v)∣=∣N1+(O,v)∣|N^+_2(O,v)|=|N^+_1(O,v)| for every vertex vv.

Converse-invariance conjecture. If

∣N1−(O,v)∣=∣N1+(O,v)∣=∣N2+(O,v)∣=k|N^-_1(O,v)|=|N^+_1(O,v)|=|N^+_2(O,v)|=k

for all veV(O)v e V(O), then

∣N2−(O,v)∣=k|N^-_2(O,v)|=k

for all v∈V(O)v\in V(O).

This would imply that the converse of every such regular Seymour-tight orientation is again Seymour-tight. Converse-invariance is known for vertex-transitive Seymour-tight orientations, but fails for general strongly connected Seymour-tight orientations; the stated weaker regularity condition remains open.

References

Primary source

Krystal Guo, Ross J. Kang and Gabriëlle Zwaneveld, “Seymour-tight orientations”, arXiv:2603.29626 (2026).

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