On the Dimension of Exceptional Parameters for Nonlinear Projections, and the Discretized Elekes-Rónyai Theorem

Orit E. Raz, Joshua Zahl*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract

We consider four related problems. (1) Obtaining dimension estimates for the set of exceptional vantage points for the pinned Falconer distance problem. (2) Nonlinear projection theorems, in the spirit of Kaufman, Bourgain, and Shmerkin. (3) The parallelizability of planar d-webs. (4) The Elekes-Rónyai theorem on expanding polynomials. Given a Borel set A in the plane, we study the set of exceptional vantage points, for which the pinned distance Δp(A) has small dimension, that is, close to (dimA)/2. We show that if this set has positive dimension, then it must have very special structure. This result follows from a more general single-scale nonlinear projection theorem, which says that if ϕ1, ϕ2, ϕ3 are three smooth functions whose associated 3-web has non-vanishing Blaschke curvature, and if A is a (δ,α)2-set in the sense of Katz and Tao, then at least one of the images ϕi(A) must have measure much larger than |A|1/2, where |A| stands for the measure of A. We prove analogous results for d smooth functions ϕ1,…,ϕd, whose associated d-web is not parallelizable. We use similar tools to characterize when bivariate real analytic functions are “dimension expanding” when applied to a Cartesian product: if P is a bivariate real analytic function, then P is either locally of the form h(a(x)+b(y)), or P(A,B) has dimension at least α+c whenever A and B are Borel sets with Hausdorff dimension α. Again, this follows from a single-scale estimate, which is an analogue of the Elekes-Rónyai theorem in the setting of the Katz-Tao discretized ring conjecture.

Original languageAmerican English
Pages (from-to)209-262
Number of pages54
JournalGeometric and Functional Analysis
Volume34
Issue number1
DOIs
StatePublished - Feb 2024

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© The Author(s), under exclusive licence to Springer Nature Switzerland AG 2024.

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