Fast winning strategies in a generalized van der Waerden game

Abstract

Consider the following Maker-Breaker game. Fix a finite subset $S\subset \mathbb{N}$ of the naturals. The players Maker and Breaker take turns choosing previously unclaimed natural numbers. Maker wins by eventually building a homothetic copy $aS+b$ of $S$, where $a\in \mathbb{N}\setminus \{0\}$ and $b\in \mathbb{Z}$. This is a generalization of the van der Waerden game analyzed by Beck. By the Hales-Jewett theorem, there exists a constant $c$ depending only on $|S|$ such that Maker can win in $c$ or less moves. We show that Maker can win in $|S|$ moves if $|S|\le 3$. When $|S|=4$, we show that Maker can always win in $5$ or less moves and describe all $S$ such that Maker can win in $4$ moves. If $|S|\ge 5$, Maker has no winning strategy in $|S|$ moves.

17 pages, 6 figures, 2 tables