# Nonexistence results for relaxation spectra with compact support

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Type Article
Original language English 035006 13 Inverse Problems 32 3 https://doi.org/10.1088/0266-5611/32/3/035006 Published - 19 Feb 2016
In this paper we consider the problem of recovering the (transformed) relaxation spectrum h from the (transformed) loss modulus g by inverting the integral equation $g={\rm{sech}}\ast h$, where $\ast$ denotes convolution, using Fourier transforms. We are particularly interested in establishing properties of h, having assumed that the Fourier transform of g has entire extension to the complex plane. In the setting of square integrable functions, we demonstrate that the Paley–Wiener theorem cannot be used to show the existence of non-trivial relaxation spectra with compact support. We prove a stronger result for tempered distributions: there are no non-trivial relaxation spectra with compact support. Finally we establish necessary and sufficient conditions for the relaxation spectrum h to be strictly positive definite.