Strong polarization and entropy

Abstract

We show that for any set of n unit vectors v_1,\ldots,v_n in a real Hilbert space and positive numbers p_1,\ldots,p_n satisfying \sum_j p_j = 1, there exists a unit vector u such that \sum_{j=1}^n \frac{p_j^2}{\langle v_j, u\rangle^2}\leq 1. This inequality is a weighted version of the strong polarization inequality. As immediate corollaries, it yields a polarization inequality for products of powers of linear functionals and a strengthening of Bang's classical plank theorem for Hilbert spaces. The proof follows the approach introduced by Martínez and Ortega-Moreno in their recent solution to the strong polarization conjecture posed by Ball and Frenkel. We further note that our weighted inequality admits a Shannon-entropy interpretation: in a random sensing model, the entropy of the weights controls the minimum expected logarithmic loss.