Weighted exponential approximation and non-classical orthogonal spectral measures

A long-standing open problem in harmonic analysis is: given a non-negative measure μ on R, find the infimal width of frequencies needed to approximate any function in L2(μ). We consider this problem in the “perturbative regime”, and characterize asymptotic smallness of perturbations of measures which do not change that infimal width. Then we apply this result to show that there are no local restrictions on the structure of orthogonal spectral measures of one-dimensional Schrödinger operators on a finite interval. This answers a question raised by V.A. Marchenko.