English

The no-$\beta$ McMullen game and the perfect set property

Logic 2021-10-08 v1

Abstract

Given a target set ARdA\subseteq \mathbb{R}^d and a real number β(0,1)\beta\in (0,1), McMullen introduced the notion of AA being an absolutely β\beta-winning set. This involves a two player game which we call the β\beta-McMullen game. We consider the version of this game in which the parameter β\beta is removed, which we call the no-β\beta McMullen game. More generally, we consider the game with respect to arbitrary norms on Rd\mathbb{R}^d, and even more generally with respect to general convex sets. We show that for strictly convex sets in Rd\mathbb{R}^d, polytopes in Rd\mathbb{R}^d, and general convex sets in R2\mathbb{R}^2, that player I\boldsymbol{I} wins the no-β\beta McMullen game iff AA contains a perfect set and player II\boldsymbol{I}\kern-0.05cm\boldsymbol{I} wins iff AA is countable. So, the no-β\beta McMullen game is equivalent to the perfect set game for AA in these cases. The proofs of these results use a connection between the geometry of the game and techniques from logic. Because of the geometry of this game, this result has strong implications for the geometry of uncountable sets in Rd\mathbb{R}^d. We also present an example of a compact, convex set in R3\mathbb{R}^3 to which our methods do not apply, and also an example due to D.\ Simmons of a closed, convex set in 2(R)\ell_2(\mathbb{R}) which illustrate the obstacles in extending the results further.

Keywords

Cite

@article{arxiv.2110.03033,
  title  = {The no-$\beta$ McMullen game and the perfect set property},
  author = {Logan Crone and Lior Fishman and Stephen Jackson},
  journal= {arXiv preprint arXiv:2110.03033},
  year   = {2021}
}