Bang-Jensen, Picasarri-Arrieta, and Yeo's characterization of champions

A tournament HH is a champion if there exists an integer cHc_H such that every HH-free tournament has acyclic dichromatic number at most cHc_H. The notation TTkTT_k denotes the transitive tournament on kk vertices, and Δ(1,1,k)TTk\Delta(1,1,k)\Rightarrow TT_k is interpreted using the tournament constructions defined in the source. Bang-Jensen, Picasarri-Arrieta, and Yeo's conjecture. A tournament HH is a champion if and only if HH is isomorphic to a subtournament of

TTk(Δ(1,1,k)TTk)TT_k \Rightarrow (\Delta(1,1,k) \Rightarrow TT_k)

for some integer k1k\geq 1. Since every champion is a hero but not every hero is a champion, this conjecture seeks a precise characterization of the tournaments whose exclusion gives a bounded acyclic dichromatic number.

Sources & referencesView supporting material

Primary source

Pierre Aboulker, Pierre Charbit, Samuel Coulomb, Kathryn Nurse and Lucas Picasarri-Arrieta, “Acyclic Dichromatic Number of Tournaments: these are the Champions”, arXiv:2607.14694 (2026).

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