New Bounds for the Same-Type Lemma

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 $\lvert Y_i\rvert \geq \frac{1}{poly(m)}\lvert X_i\rvert$ such that, for $y_1\in Y_1,\dotsc,y_m\in Y_m$, the orientations of the\linebreak $(d+1)$-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.