Projections of planar sets in well-separated directions (1504.07189v5)

Abstract: First, let $K \subset B(0,1) \subset \mathbb{R}^{2}$ be a set with $\mathcal{H}{\infty}^{1}(K) \sim 1$, and write $\pi{e}(K)$ for the orthogonal projection of $K$ into the line spanned by $e \in S^{1}$. For $1/2 \leq s < 1$, write $$E_{s} := {e : N(\pi_{e}(K),\delta) \leq \delta^{-s}}, $$ where $N(A,r)$ is the $r$-covering number of the set $A$. It is well-known -- and essentially due to R. Kaufman -- that $N(E_{s},\delta) \lessapprox \delta^{-s}$. Using the polynomial method, I prove that $$ N(E_{s},r) \lessapprox \min\left{\delta^{-s}\left(\frac{\delta}{r}\right)^{1/2},r^{-1}\right}, \quad \delta \leq r \leq 1.$$ I construct examples showing that the exponents in the bound are sharp for $\delta \leq r \leq \delta^{s}$. The second theorem concerns projections of $1$-Ahlfors-David regular sets. Let $A \geq 1$ and $1/2 \leq s < 1$ be given. I prove that, for $p = p(A,s) \in \mathbb{N}$ large enough, the finite set of unit vectors $S_{p} := {e^{2\pi i k/p} : 0 \leq k < p}$ has the following property. If $K \subset B(0,1)$ is non-empty and $1$-Ahlfors-David regular with regularity constant at most $A$, then $$\frac{1}{p} \sum_{e \in S_{p}} N(\pi_{e}(K),\delta) \geq \delta^{-s}$$ for all small enough $\delta > 0$. In particular, $\overline{\dim}{\text{B}} \pi{e}(K) \geq s$ for some $e \in S_{p}$.