The Maximum Codimension of a Salem Submanifold

Abstract

We determine a geometric condition necessary and sufficient for an $m$-dimensional manifold in Euclidean space to support a probability measure $μ$ satisfying the Fourier decay bound $|\widehatμ(ζ)| \lesssim_\varepsilon |ζ|^{\varepsilon - m/2}$ for all $\varepsilon > 0$. As a result, for each $m > 0$, we explicitly determine the largest codimension of an $m$-dimensional smooth submanifold $M$ of Euclidean space which is a Salem set. This largest codimension is precisely expressible in terms of the Radon-Hurwitz numbers. In particular, we find that the only odd dimensional manifolds which can be Salem sets are hypersurfaces, and that the largest codimension of an $m$-dimensional manifold which is a Salem set is upper bounded by $2 \lg(m/2) + 3$, and equal to $2 \lg(m/2) + 3$ when $m$ is a power of 16. The proof strategy, which involves covering manifolds by certain stationary sets associated with the Fourier transform on that manifold, is robust, and we demonstrate its use by proving that all nondegenerate curves in $\mathbf{R}^n$ have Fourier dimension equal to $2/n$, and find an alternate proof of a result of Junjie Zhu on the Fourier dimension of hypersurfaces with a fixed number of vanishing principal curvatures.