# Induced Ramsey-type results and binary predicates for point sets

## TL;DR

This paper explores Ramsey-type properties of finite point sets in the plane, introducing new classes of point sets with specific monochromatic and order-type properties, and demonstrates limitations on local orientation-determining functions.

## Contribution

The authors extend known results by constructing new $(k,2)$-Ramsey point sets and prove the non-existence of local functions that determine point triple orientations.

## Key findings

- Introduced a new family of $(k,2)$-Ramsey point sets.
- Proved that no local orientation function can exist for certain point sets.
- Extended previous results on Ramsey properties of point sets.

## Abstract

Let $k$ and $p$ be positive integers and let $Q$ be a finite point set in general position in the plane. We say that $Q$ is $(k,p)$-Ramsey if there is a finite point set $P$ such that for every $k$-coloring $c$ of $\binom{P}{p}$ there is a subset $Q'$ of $P$ such that $Q'$ and $Q$ have the same order type and $\binom{Q'}{p}$ is monochromatic in $c$. Ne\v{s}et\v{r}il and Valtr proved that for every $k \in \mathbb{N}$, all point sets are $(k,1)$-Ramsey. They also proved that for every $k \ge 2$ and $p \ge 2$, there are point sets that are not $(k,p)$-Ramsey. As our main result, we introduce a new family of $(k,2)$-Ramsey point sets, extending a result of Ne\v{s}et\v{r}il and Valtr. We then use this new result to show that for every $k$ there is a point set $P$ such that no function $\Gamma$ that maps ordered pairs of distinct points from $P$ to a set of size $k$ can satisfy the following "local consistency" property: if $\Gamma$ attains the same values on two ordered triples of points from $P$, then these triples have the same orientation. Intuitively, this implies that there cannot be such a function that is defined locally and determines the orientation of point triples.