Incidences in Three Dimensions and Distinct Distances in the Plane

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 $\reals<sup>3$. Applying these bounds, we obtain, among several other results, the upper bound $O(s<sup>3)$ 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 $O(s<sup>3/k<sup>{12/7})$. One of our unresolved conjectures is that this number is $O(s<sup>3/k<sup>2)$, 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.