English

Projections of planar sets in well-separated directions

Classical Analysis and ODEs 2016-04-21 v5

Abstract

First, let KB(0,1)R2K \subset B(0,1) \subset \mathbb{R}^{2} be a set with H1(K)1\mathcal{H}_{\infty}^{1}(K) \sim 1, and write πe(K)\pi_{e}(K) for the orthogonal projection of KK into the line spanned by eS1e \in S^{1}. For 1/2s<11/2 \leq s < 1, write Es:={e:N(πe(K),δ)δs},E_{s} := \{e : N(\pi_{e}(K),\delta) \leq \delta^{-s}\}, where N(A,r)N(A,r) is the rr-covering number of the set AA. It is well-known -- and essentially due to R. Kaufman -- that N(Es,δ)δsN(E_{s},\delta) \lessapprox \delta^{-s}. Using the polynomial method, I prove that N(Es,r)min{δs(δr)1/2,r1},δr1. 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 δrδs\delta \leq r \leq \delta^{s}. The second theorem concerns projections of 11-Ahlfors-David regular sets. Let A1A \geq 1 and 1/2s<11/2 \leq s < 1 be given. I prove that, for p=p(A,s)Np = p(A,s) \in \mathbb{N} large enough, the finite set of unit vectors Sp:={e2πik/p:0k<p}S_{p} := \{e^{2\pi i k/p} : 0 \leq k < p\} has the following property. If KB(0,1)K \subset B(0,1) is non-empty and 11-Ahlfors-David regular with regularity constant at most AA, then 1peSpN(πe(K),δ)δs\frac{1}{p} \sum_{e \in S_{p}} N(\pi_{e}(K),\delta) \geq \delta^{-s} for all small enough δ>0\delta > 0. In particular, dimBπe(K)s\overline{\dim}_{\text{B}} \pi_{e}(K) \geq s for some eSpe \in S_{p}.

Keywords

Cite

@article{arxiv.1504.07189,
  title  = {Projections of planar sets in well-separated directions},
  author = {Tuomas Orponen},
  journal= {arXiv preprint arXiv:1504.07189},
  year   = {2016}
}

Comments

20 pages, 3 figures. v5: Corrected typos and updated references. To appear in Adv. Math