English

Above and below

Combinatorics 2026-05-27 v1

Abstract

We study a family of above-below Ramsey functions AB(d)(k)\operatorname{AB}^{(d)}(k) defined for sequences of points in Rd\mathbb R^d whose projections to Rd1\mathbb R^{d-1} have cyclic order type. The case d=3d=3 is the above-below function AB(k)\operatorname{AB}(k) that was first introduced by Pohoata and Zakharov in their work on the Erd\H{o}s-Szekeres problem in R3\mathbb{R}^{3}. We prove the sharp estimate AB(k)=22Θ(k), \operatorname{AB}(k)=2^{2^{\Theta(k)}}, and, more generally, show that AB(d)(k)\operatorname{AB}^{(d)}(k) is closely related to the higher-order cup-cap function of Eli\'a\v{s} and Matou\v{s}ek and the monotone Ramsey numbers of Balko.

Cite

@article{arxiv.2605.27061,
  title  = {Above and below},
  author = {Wenchong Chen and Cosmin Pohoata},
  journal= {arXiv preprint arXiv:2605.27061},
  year   = {2026}
}