Eigenfunctions, free boundaries, and time-frequency localization

Abstract

We develop an inverse theory for time--frequency localization operators, formulated as a free-boundary problem in which the localization domain is unknown and its boundary is recovered from prescribed spectral data. The guiding principle is that an eigenfunction can itself be regarded as geometric data determining a localization domain, with strong consequences for the associated variational problem. We obtain five main results. First, for every polynomial $f_0$ sufficiently close to the Gaussian and every $\lambda\in(0,1)$, we construct a real-analytic domain $U_\lambda$ such that $f_0$ is an eigenfunction of the corresponding localization operator with eigenvalue $\lambda$, providing a general inverse construction of localization domains. Second, we recover the null-set invariant Abreu--D"orfler characterization of disks as the only simply connected localization domains having a Hermite polynomial as eigenfunction. Third, we show that simple connectivity is essential: a weighted Hele--Shaw flow yields an infinite-dimensional family of non-radial, doubly connected analytic domains for which the Gaussian is an eigenfunction. Fourth, we prove the optimality of the exponent $1/2$ in the quantitative stability inequality of G'omez--Guerra--Ramos--Tilli. Finally, we prove that every local maximizer of the Gaussian Faber--Krahn problem is a disk and, using a Fock-space concentration--compactness profile decomposition, obtain a new proof of the Nicola--Tilli concentration theorem.