English

Online Ramsey turnaround numbers

Combinatorics 2025-12-10 v1

Abstract

The online Ramsey turnaround game is a game between two players, Builder and Painter, on a board of nn vertices using 33 colors, for a fixed graph HH on at most nn vertices. The goal of Painter is to force a monochromatic copy of HH, the goal of Builder is to avoid this as long as possible. In each round of the game, Builder exposes one new edge and is allowed to forbid the usage of one color for Painter to color this newly exposed edge, and Painter colors the edge according to this restriction. The game is over as soon as Painter manages to achieve a monochromatic copy of HH. For sufficiently large nn, we consider the smallest number f(n,H)f(n, H) of edges so that Painter can always win after f(n,H)f(n, H) edges have been exposed by Builder. In addition, we define f(H)f(H) to be the smallest nn such that Painter can always win on a clique with nn vertices. We give bounds for both functions and show that this problem is closely related to other concepts in extremal graph theory, such as polychromatic colorings, set-coloring Ramsey numbers, chromatic Ramsey numbers, and 2-color Tur\'an numbers.

Keywords

Cite

@article{arxiv.2512.08053,
  title  = {Online Ramsey turnaround numbers},
  author = {Nóra Almási and Maria Axenovich},
  journal= {arXiv preprint arXiv:2512.08053},
  year   = {2025}
}

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R2 v1 2026-07-01T08:15:46.396Z