# Title: Hausdorff-dimension analogue of the Elekes—Rónyai theorem and related problems

## Speaker: Orit Raz, Hebrew University and the Institute for
Advanced Study

## Date and time: 6pm (New York time), Tuesday, April 25, 2023

## Place: WWH1314 (room 1314 Warren Weaver Hall, 251 Mercer Street)

### ...and on Zoom, details on the seminar mailing list

For a real bivariate polynomial $f$ and finite set $A$ and $B$ of
cardinality $n$, Elekes and Rónyai proved that either $f(A\times B)$
is much larger than $n$, or $f$ has a very specific form (essentially,
$f(x,y)=x+y$). In the talk, I will tell about an analogue of this
problem, where $A$ and $B$ are now Borel subsets of $\mathbb{R}$, each
of Hausdorff dimension $\alpha$. In a recent work, joint with Josh
Zahl, we prove that in this case $f(A\times B)$ will have Hausdorff
dimension at least $\alpha+c$, where $c=c(\alpha)>0$, unless $f$ has
the specific special form as above.

I will show a connection between the Elekes—Rónyai theorem just
mentioned and the following result: Given a set $E$ in the plane,
consider the set of exceptional points, for which the pinned distance
set $\Delta_p(E)$ has small Hausdorff dimension, that is, close to
${\rm dim}(A)/2$. If this set has a positive dimension, then it must
have a special structure. As a corollary one deduces a certain
instance of the pinned Falconer’s distance problem.

For the proofs we apply a reduction to the discretized setting
introduced by Katz and Tao. In the discretized setting, our proofs are
inspired by their counterparts in the finite case, where the classical
Szemerédi and Trotter incidence bound is replaced by a recent result
of Shmerkin.

The talk is based on a joint work with Josh Zahl.