Ramsey-type results for semi-algebraic relations

Abstract

A k-ary semi-algebraic relation E on ℝ_d is a subset of ℝ_(kd), the set of k-tuples of points in ℝ_(d), which is determined by a finite number of polynomial inequalities in kd real variables. The description complexity of such a relation is at most t if d, k ≤ t and the number of polynomials and their degrees are all bounded by t. A set A ⊂ ℝ_d is called homogeneous if all or none of the k-tuples from A satisfy E. A large number of geometric Ramsey-type problems and results can be formulated as questions about finding large homogeneous subsets of sets in ℝ_d equipped with semi-algebraic relations. In this paper, we study Ramsey numbers for k-ary semi-algebraic relations of bounded complexity and give matching upper and lower bounds, showing that they grow as a tower of height k − 1. This improves upon a direct application of Ramsey's theorem by one exponential and extends a result of Alon, Pach, Pinchasi, Radoičić, and Sharir, who proved this for k = 2. We apply our results to obtain new estimates for some geometric Ramsey-type problems relating to order types and one-sided sets of hyperplanes. We also study the off-diagonal case, achieving some partial results.

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