Real Paley–wiener Theorems and Local Spectral Radius Formulas

We systematically develop real Paley–Wiener theory for the Fourier transform on Rd for Schwartz functions, Lp-functions and distributions, in an elementary treatment based on the inversion theorem. As an application, we show how versions of classical Paley–Wiener theorems can be derived from the real ones via an approach which does not involve domain shifting and which may be put to good use for other transforms of Fourier type as well. An explanation is also given as to why the easily applied classical Paley–Wiener theorems are unlikely to be able to yield information about the support of a function or distribution which is more precise than giving its convex hull, whereas real Paley–Wiener theorems can be used to reconstruct the support precisely, albeit at the cost of combinatorial complexity. We indicate a possible application of real Paley–Wiener theory to partial differential equations in this vein, and furthermore we give evidence that a number of real Paley– Wiener results can be expected to have an interpretation as local spectral radius formulas. A comprehensive overview of the literature on real Paley– Wiener theory is included.