Fässler–Orponen projection conjecture for non-degenerate curves on the sphere

Let γ:[0,1]S2\gamma:[0,1]\to\mathbb S^2 be a C2C^2 curve satisfying

det(γ,γ,γ)(θ)0,θ[0,1].\det(\gamma,\gamma',\gamma”)(\theta)\ne 0,\quad \forall\,\theta\in[0,1].

For each θ\theta, let pγ(θ)p_{\gamma(\theta)} denote projection onto the line in direction γ(θ)\gamma(\theta), and let πγ(θ)\pi_{\gamma(\theta)} denote projection onto the plane orthogonal to γ(θ)\gamma(\theta). Fässler–Orponen's conjecture. For every Borel set AR3A\subseteq\mathbb R^3,

dimpγ(θ)(A)=min{1,dimA},dimπγ(θ)(A)=min{2,dimA}\dim p_{\gamma(\theta)}(A)=\min\{1,\dim A\},\qquad \dim \pi_{\gamma(\theta)}(A)=\min\{2,\dim A\}

both hold for L1\mathcal L^1-almost every θ[0,1]\theta\in[0,1]. This is a restricted projection conjecture for directions lying on a non-degenerate curve on the sphere; the supplied text attributes it to Fässler and Orponen but gives no resolution, so its status is left open.

Sources & referencesView supporting material

Primary source

Tainara Borges, Siddharth Mulherkar and Tongou Yang, “Study guide for "On restricted projections to planes in R^3"”, arXiv:2403.17989 (2024).

Additional references

2 papers in this index state this conjecture (2021–2024). The statement above is taken from the most recent of them; the others are arXiv:2107.14701.

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