New bounds for the same-type lemma

TL;DR

This paper establishes new bounds for the same-type lemma in finite sets in Euclidean space, showing the existence of large subsets with orientation-invariant tuples, and demonstrates the necessity of polynomial dependence on the number of sets.

Contribution

It generalizes the same-type lemma to multiple sets, provides bounds on subset sizes, and introduces an algorithm to approximate optimal constants.

Findings

Existence of large subsets with orientation-invariant tuples

Polynomial dependence on the number of sets is necessary

An algorithm approximates the best possible constants

Abstract

Given finite sets $X_1,\dotsc,X_m$ in $\mathbb{R}^d$ (with $d$ fixed), we prove that there are respective subsets $Y_1,\dotsc,Y_m$ with $|Y_i|\ge \frac{1}{\operatorname{poly}(m)}|X_i|$ such that, for $y_1\in Y_1,\dotsc,y_m\in Y_m$, the orientations of the $(d+1)$-tuples from $y_1,\dotsc,y_m$ do not depend on the actual choices of points $y_1,\dotsc,y_m$. 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.