Kaufman and Falconer estimates for radial projections and a continuum version of Beck's Theorem (2209.00348v1)

Abstract: We provide several new answers on the question: how do radial projections distort the dimension of planar sets? Let $X,Y \subset \mathbb{R}^{2}$ be non-empty Borel sets. If $X$ is not contained on any line, we prove that [ \sup_{x \in X} \dim_{\mathrm{H}} \pi_{x}(Y) \geq \min{\dim_{\mathrm{H}} X,\dim_{\mathrm{H}} Y,1}. ] If $\dim_{\mathrm{H}} Y > 1$, we have the following improved lower bound: [ \sup_{x \in X} \dim_{\mathrm{H}} \pi_{x}(Y \, \setminus \, {x}) \geq \min{\dim_{\mathrm{H}} X + \dim_{\mathrm{H}} Y - 1,1}. ] Our results solve conjectures of Lund-Thang-Huong, Liu, and the first author. Another corollary is the following continuum version of Beck's theorem in combinatorial geometry: if $X \subset \mathbb{R}^{2}$ is a Borel set with the property that $\dim_{\mathrm{H}} (X \, \setminus \, \ell) = \dim_{\mathrm{H}} X$ for all lines $\ell \subset \mathbb{R}^{2}$, then the line set spanned by $X$ has Hausdorff dimension at least $\min{2\dim_{\mathrm{H}} X,2}$. While the results above concern $\mathbb{R}^{2}$, we also derive some counterparts in $\mathbb{R}^{d}$ by means of integralgeometric considerations. The proofs are based on an $\epsilon$-improvement in the Furstenberg set problem, due to the two first authors, a bootstrapping scheme introduced by the second and third author, and a new planar incidence estimate due to Fu and Ren.