Regularization of inverse problems by an approximate matrix-function technique

In this work, we introduce and investigate a class of matrix-free regularization techniques for discrete linear ill-posed problems based on the approximate computation of a special matrix function. In order to produce a regularized solution, the proposed strategy employs a regular approximation of the Heavyside step function computed into asmallKrylov subspace. This particular feature allows our proposal to be independent of the structure of the underlying matrix. If on the one hand, the use of the Heaviside step function prevents the amplification of the noise by suitably filtering the responsible components of the spectrum of the discretization matrix, on the other hand, it permits the correct reconstruction of the signal inverting the remaining part of the spectrum. Numerical tests on a gallery of standard benchmark problems are included to prove the efficacy of our approach even for problems affected by a high level of noise.