“…As a common feature of the two operators, for their compactness [1,6], the relative singular values cluster to zero asymptotically (i.e., as their index tends to infinity); this entails that the related inversion problem at hand is ill-posed. Accordingly, in order to obtain a stable inversion of the operators in (1) and (3), the singular spectrum should be suitably truncated by accounting for the noise level on data [6,8].…”