On sets defining few ordinary hyperplanes

TL;DR

This paper establishes tight bounds on the minimum number of ordinary hyperplanes determined by large point sets in high-dimensional projective spaces, characterizes the structure of sets with few such hyperplanes, and solves a higher-dimensional orchard problem.

Contribution

It extends the theory of ordinary hyperplanes to higher dimensions, providing exact bounds, a structure theorem, and solving a related combinatorial geometry problem.

Findings

Minimum number of ordinary hyperplanes is inom{n-1}{d-1} - O_d(n^{\u230a(d-1)/2b}) for large n.

Sets with few ordinary hyperplanes are mostly contained in special algebraic curves or hyperplanes.

Maximum number of (d+1)-point hyperplanes is determined, solving a higher-dimensional orchard problem.

Abstract

Let $P$ be a set of $n$ points in real projective $d$-space, not all contained in a hyperplane, such that any $d$ points span a hyperplane. An ordinary hyperplane of $P$ is a hyperplane containing exactly $d$ points of $P$. We show that if $d\ge 4$, the number of ordinary hyperplanes of $P$ is at least $\binom{n-1}{d-1} - O_d(n^{\lfloor(d-1)/2\rfloor})$ if $n$ is sufficiently large depending on $d$. This bound is tight, and given $d$, we can calculate the exact minimum number for sufficiently large $n$. This is a consequence of a structure theorem for sets with few ordinary hyperplanes: For any $d \ge 4$ and $K > 0$, if $n \ge C_d K^8$ for some constant $C_d > 0$ depending on $d$ and $P$ spans at most $K\binom{n-1}{d-1}$ ordinary hyperplanes, then all but at most $O_d(K)$ points of $P$ lie on a hyperplane, an elliptic normal curve, or a rational acnodal curve. We also find the maximum number of $(d+1)$-point hyperplanes, solving a $d$-dimensional analogue of the orchard problem. Our proofs rely on Green and Tao's results on ordinary lines, our earlier work on the $3$-dimensional case, as well as results from classical algebraic geometry.