“…This problem has been investigated by several authors in recent years, for example in [4,5,6,16,26]. It has a wide range of applications, for example in optical tomography [3], antenna control [18] and in the acoustic reconstruction part of several hybrid imaging methods, most notably photo-and thermoacoustic tomography [2,19], see also [1,6]. The single-frequency ISP, i.e., when one only has a measurement at a single frequency, is not uniquely solvable due to F k having a large kernel.…”