Solving Abel integral equations by regularisation in Hilbert scales

TL;DR

This paper develops a regularisation method in Hilbert scales for solving Abel integral equations, generalising previous results to arbitrary order and providing a numerical approach with confirmed convergence.

Contribution

It introduces a new Hilbert scale framework for Abel integral equations, extending regularisation techniques to any order and general kernels, with practical numerical implementation.

Findings

The method achieves theoretical convergence rates.

Numerical examples confirm the effectiveness of the approach.

Applicable to a wide range of kernels and orders.

Abstract

Integral operators of Abel type of order a > 0 arise naturally in a large spectrum of physical processes. Their inversion requires care since the resulting inverse problem is ill-posed. The purpose of this work is to devise and analyse a family of appropriate Hilbert scales so that the operator is ill-posed of order a in the scale. We provide weak regularity assumptions on the kernel underlying the operator for the above to hold true. Our construction leads to a well-defined regularisation strategy by Tikhonov regularisation in Hilbert scales. We thereby generalise the results of Gorenflo and Yamamoto for a < 1 to arbitrary a > 0 and more general kernels. Thanks to tools from interpolation theory, we also show that the a priori associated to the Hilbert scale formulates in terms of smoothness in usual Sobolev spaces up to boundary conditions, and that the regularisation term actually amounts to penalising derivatives. Finally, following the theoretical construction, we develop a comprehensive numerical approach, where the a priori is encoded in a single parameter rather than in a full operator. Several numerical examples are shown, both confirming the theoretical convergence rates and showing the general applicability of the method.