Lower bounds on geometric Ramsey functions

We continue a sequence of recent works studying Ramsey functions for semialgebraic predicates in $\mathbb{R}^d$. A $k$-ary semialgebraic predicate $\Phi(x_1,\ldots,x_k)$ on $\mathbb{R}^d$ is a Boolean combination of polynomial equations and inequalities in the $kd$ coordinates of $k$ points $x_1,\ldots,x_k\in\mathbb{R}^d$. A sequence $P=(p_1,\ldots,p_n)$ of points in $\mathbb{R}^d$ is called $\Phi$-homogeneous if either $\Phi(p_{i_1}, \ldots,p_{i_k})$ holds for all choices $1\le i_1 < \cdots < i_k\le n$, or it holds for no such choice. The Ramsey function $R_\Phi(n)$ is the smallest $N$ such that every point sequence of length $N$ contains a $\Phi$-homogeneous subsequence of length $n$. Conlon, Fox, Pach, Sudakov, and Suk constructed the first examples of semialgebraic predicates with the Ramsey function bounded from below by a tower function of arbitrary height: for every $k\ge 4$, they exhibit a $k$-ary $\Phi$ in dimension $2^{k-4}$ with $R_\Phi$ bounded below by a tower of height $k-1$. We reduce the dimension in their construction, obtaining a $k$-ary semialgebraic predicate $\Phi$ on $\mathbb{R}^{k-3}$ with $R_\Phi$ bounded below by a tower of height $k-1$. We also provide a natural geometric Ramsey-type theorem with a large Ramsey function. We call a point sequence $P$ in $\mathbb{R}^d$ order-type homogeneous if all $(d+1)$-tuples in $P$ have the same orientation. Every sufficiently long point sequence in general position in $\mathbb{R}^d$ contains an order-type homogeneous subsequence of length $n$, and the corresponding Ramsey function has recently been studied in several papers. Together with a recent work of B\'ar\'any, Matou\v{s}ek, and P\'or, our results imply a tower function of $\Omega(n)$ of height $d$ as a lower bound, matching an upper bound by Suk up to the constant in front of $n$.Comment: 12 page

Curves in R^d intersecting every hyperplane at most d+1 times

By a curve in R^d we mean a continuous map gamma:I -> R^d, where I is a closed interval. We call a curve gamma in R^d at most k crossing if it intersects every hyperplane at most k times (counted with multiplicity). The at most d crossing curves in R^d are often called convex curves and they form an important class; a primary example is the moment curve {(t,t^2,...,t^d):t\in[0,1]}. They are also closely related to Chebyshev systems, which is a notion of considerable importance, e.g., in approximation theory. We prove that for every d there is M=M(d) such that every at most d+1 crossing curve in R^d can be subdivided into at most M convex curves. As a consequence, based on the work of Elias, Roldan, Safernova, and the second author, we obtain an essentially tight lower bound for a geometric Ramsey-type problem in R^d concerning order-type homogeneous sequences of points, investigated in several previous papers.Comment: Corrected proof of Lemma 3.