An optimal plank theorem

Abstract

We give a new proof of Fejes Tóth’s zone conjecture: for any sequence v 1 , v 2 , … , v n v_1,v_2,\dots ,v_n of unit vectors in a real Hilbert space H \mathcal {H} , there exists a unit vector v v in H \mathcal {H} such that | ⟨ v k , v ⟩ | ≥ sin ⁡ ( π / 2 n ) \begin{equation*} |\langle v_k,v \rangle | \geq \sin (\pi /2n) \end{equation*} for all k k . This can be seen as a sharp version of the plank theorem for real Hilbert spaces. Our approach is inspired by Ball’s solution to the complex plank problem and thus unifies both the complex and the real solution under the same method.