We study the optimal approximation of the solution of an operator equation Au=f by linear mappings of rank n and compare this with the best n-term approximation with respect to an optimal Riesz basis. We consider worst case errors, where f is an element of the unit ball of a Hilbert space. We apply our results to boundary value problems for elliptic PDEs on an arbitrary bounded Lipschitz domain. Here we prove that approximation by linear mappings is as good as the best n-term approximation with respect to an optimal Riesz basis. Our results are concerned with approximation, not with computation. Our goal is to understand better the possibilities of nonlinear approximation.
@InProceedings{novak_et_al:DagSemProc.04401.12, author = {Novak, Erich and Dahlke, Stephan and Sickel, Winfried}, title = {{Optimal Approximation of Elliptic Problems by Linear and Nonlinear Mappings}}, booktitle = {Algorithms and Complexity for Continuous Problems}, series = {Dagstuhl Seminar Proceedings (DagSemProc)}, ISSN = {1862-4405}, year = {2005}, volume = {4401}, editor = {Thomas M\"{u}ller-Gronbach and Erich Novak and Knut Petras and Joseph F. Traub}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/DagSemProc.04401.12}, URN = {urn:nbn:de:0030-drops-1471}, doi = {10.4230/DagSemProc.04401.12}, annote = {Keywords: elliptic operator equation , worst case error , linear approximation method , nonlinear approximation method , best n-term approximation Bernstein widths , manifold widths} }
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