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# Journal of Inverse and Ill-posed Problems

Editor-in-Chief: Kabanikhin, Sergey I.

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# On ill-posedness concepts, stable solvability and saturation

Bernd Hofmann
/ Robert Plato
Published Online: 2017-12-23 | DOI: https://doi.org/10.1515/jiip-2017-0090

## Abstract

We consider different concepts of well-posedness and ill-posedness and their relations for solving nonlinear and linear operator equations in Hilbert spaces. First, the concepts of Hadamard and Nashed are recalled which are appropriate for linear operator equations. For nonlinear operator equations, stable respective unstable solvability is considered, and the properties of local well-posedness and ill-posedness are investigated. Those two concepts consider stability in image space and solution space, respectively, and both seem to be appropriate concepts for nonlinear operators which are not onto and/or not, locally or globally, injective. Several example situations for nonlinear problems are considered, including the prominent autoconvolution problems and other quadratic equations in Hilbert spaces. It turns out that for linear operator equations, well-posedness and ill-posedness are global properties valid for all possible solutions, respectively. The special role of the nullspace is pointed out in this case. Finally, non-injectivity also causes differences in the saturation behavior of Tikhonov and Lavrentiev regularization of linear ill-posed equations. This is examined at the end of this study.

MSC 2010: 47A52; 47J06; 65J20

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Accepted: 2017-12-13

Published Online: 2017-12-23

Published in Print: 2018-04-01

Funding Source: Deutsche Forschungsgemeinschaft

Award identifier / Grant number: HO 1454/10-1

The first author gratefully acknowledges support by the German Research Foundation (DFG) under grant HO 1454/10-1.

Citation Information: Journal of Inverse and Ill-posed Problems, Volume 26, Issue 2, Pages 287–297, ISSN (Online) 1569-3945, ISSN (Print) 0928-0219,

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