Curious ill-posedness phenomena in the composition of non-compact linear operators in Hilbert spaces

Abstract

We consider the composition of operators with non-closed range in Hilbert spaces and how the nature of ill-posedness is affected by their composition. Specifically, we study the \mbox{Hausdorff-,} Cesà ro-, integration operator, and their adjoints, as well as some combinations of those. For the composition of the Hausdorff- and the Cesà ro-operator, we give estimates of the decay of the corresponding singular values. As a curiosity, this provides also an example of two practically relevant non-compact operators, for which their composition is compact. Furthermore, we characterize those operators for which a composition with a non-compact operator gives a compact one.

Section 5 (Numerics) and Section 6 (Characterization of operators that give compact composition) were added