Incidences in Three Dimensions and Distinct Distances in the Plane

TL;DR

This paper connects the problem of counting distinct distances in a plane to incidence geometry in three dimensions, using algebraic methods to derive bounds on incidences and rotations mapping point sets.

Contribution

It introduces a novel reduction from planar distance problems to 3D incidence problems and applies algebraic techniques to bound incidences and rotations, advancing understanding of geometric configurations.

Findings

Bound on rotations mapping points: O(s^3) for three points

Incidence bounds between helices/parabolas and points in 3D

Conjecture that improved bounds imply lower bounds on distinct distances

Abstract

We first describe a reduction from the problem of lower-bounding the number of distinct distances determined by a set $S$ of $s$ points in the plane to an incidence problem between points and a certain class of helices (or parabolas) in three dimensions. We offer conjectures involving the new setup, but are still unable to fully resolve them. Instead, we adapt the recent new algebraic analysis technique of Guth and Katz \cite{GK}, as further developed by Elekes et al. \cite{EKS}, to obtain sharp bounds on the number of incidences between these helices or parabolas and points in $\reals^3$. Applying these bounds, we obtain, among several other results, the upper bound $O(s^3)$ on the number of rotations (rigid motions) which map (at least) three points of $S$ to three other points of $S$. In fact, we show that the number of such rotations which map at least $k\ge 3$ points of $S$ to $k$ other points of $S$ is close to $O(s^3/k^{12/7})$. One of our unresolved conjectures is that this number is $O(s^3/k^2)$, for $k\ge 2$. If true, it would imply the lower bound $\Omega(s/\log s)$ on the number of distinct distances in the plane.