Curves in the Fourier zeros of polytopal regions and the Pompeiu problem

Abstract

We prove that any finite union P of interior-disjoint polytopes in \(\mathbb R^d\) has the Pompeiu property, a result first proved by Williams [15]. This means that if a continuous function f on \(\mathbb R^d\) integrates to 0 on any congruent copy of \(P\) then \(f\) is identically 0. By a fundamental result of Brown, Schreiber and Taylor [4], this is equivalent to showing that the Fourier–Laplace transform of the indicator function of P does not vanish identically on any 0-centered complex sphere in \(\mathbb C^d\). Our proof initially follows the recent one of Machado and Robins [12] who are using the Brion–Barvinok formula for the Fourier–Laplace transform of a polytope. But we simplify this method considerably by removing the use of properties of Bessel function zeros. Instead we use some elementary arguments on the growth of linear combinations of exponentials with rational functions as coefficients. Our approach allows us to prove the non-existence of complex spheres of any center in the zero-set of the Fourier–Laplace transform. The planar case is even simpler in that we do not even need the Brion–Barvinok formula. We then go further in the question of which sets can be contained in the null set of the Fourier–Laplace transform of a polytope by extending results of Engel [7] who showed that rationally parametrized hypersurfaces, under some mild conditions, cannot be contained in this null-set. We show that a rationally parametrized curve which is not contained in an affine hyperplane in \(\mathbb C^d\) cannot be contained in this null-set. Results about curves parametrized by meromorphic functions are also given.