High-chromatic tournament parts without a complete pair of cyclic triangles

@misc{chen2026highchromatic,
  title = {{High-chromatic tournament parts without a complete pair of cyclic triangles}},
  author = {Guantao Chen and Hein van der Holst and Shengze Wang},
  howpublished = {arXiv},
  year = {2026},
  month = oct,
  doi = {10.48550/arxiv.2610.37920},
  eprint = {2610.37920},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2610.37920},
}

TY  - UNPB
TI  - High-chromatic tournament parts without a complete pair of cyclic triangles
AU  - Guantao Chen
AU  - Hein van der Holst
AU  - Shengze Wang
PY  - 2026
DA  - 2026/10/27
PB  - arXiv
DO  - 10.48550/arxiv.2610.37920
UR  - https://arxiv.org/abs/2610.37920
AB  - Nguyen, Scott, and Seymour conjectured that any two disjoint subtournaments of sufficiently large chromatic number contain cyclic triangles, one in each subtournament, with all edges between them oriented in the same direction. We disprove this conjecture by constructing, for every positive integer d, a tournament T with a vertex partition (X,Y) such that χ(T[X]),χ(T[Y])≥ d, but no cyclic triangle in either part is complete to a cyclic triangle in the other. Our construction combines a graph of large girth and large chromatic number with a variant of Kneser graphs defined in terms of binary functions. We give two proofs of the required chromatic lower bound, one using the Borsuk--Ulam theorem and the other using the octahedral form of Tucker's lemma.
ER  -