New bounds for the same-type lemma

Abstract

Given finite sets $X_1,\dots ,X_m$ in ${\mathbb{R}}^d$ (with $d$ fixed), we prove that there are respective subsets $Y_1,\dots ,Y_m$ with $|Y_i|\ge \frac{1}{\mathrm{poly}(m)}|X_i|$ such that, for $y_1\in Y_1,\dots ,y_m\in Y_m$, the orientations of the $(d+1)$-tuples from $y_1,\dots ,y_m$ do not depend on the actual choices of points $y_1,\dots ,y_m$. This generalizes previously known case when all the sets $X_i$ are equal. Furthermore, we give a construction showing that polynomial dependence on $m$ is unavoidable, as well as an algorithm that approximates the best-possible constants in this result.

9 pages