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arXiv · 2607.18047

Towards a strengthening of the second neighborhood conjecture

Yandong Bai·Binlong Li·Boram Park
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Abstract

A longstanding conjecture of Seymour, called Seymour's second neighborhood conjecture, states that every oriented graph $D$ contains a vertex $x$ with $|N^{++}_D(x)|\geq |N^{+}_D(x)|$. The conjecture was verified in a few special classes of oriented graphs, and it remains open for general oriented graphs. We study a stronger property, asking for a vertex $x$ such that there exists a complete matching from $N^+_D(x)$ to $N^{++}_D(x)$. We prove that this stronger version holds for every oriented graph with minimum out-degree at most $5$, and also for every $5$-anti-transitive oriented graph. This implies that every oriented planar graph satisfies the stronger version.

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Yandong Bai, Binlong Li, Boram Park. 2026-07-20. Towards a strengthening of the second neighborhood conjecture. https://arxiv.org/abs/2607.18047

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