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Preprint 25-2014


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Author(s) : Philipp Grohs , Sandra Keiper , Gitta Kutyniok , Martin Schäfer

Preprint series of the Institute of Mathematics, Technische Universität Berlin
Preprint 25-2014

MSC 2000

42C40 Wavelets
41A25 Rate of convergence, degree of approximation

Abstract :
Within the area of applied harmonic analysis, various multiscale systems such as wavelets, ridgelets, curvelets, and shearlets have been introduced and successfully applied. The key property of each of those systems are their (optimal) approximation properties in terms of the decay of the L2-error of the best N-term approximation for a certain class of functions. In this paper, we introduce the general framework of α-molecules, which encompasses most multiscale systems from applied harmonic analysis, in particular, wavelets, ridgelets, curvelets, and shearlets as well as extensions of such with α being a parameter measuring the degree of anisotropy, as a means to allow a unified treatment of approximation results within this area. Based on an α-scaled index distance, we first prove that two systems of α-molecules are almost orthogonal. This leads to a general methodology to transfer approximation results within this framework, provided that certain consistency and time-frequency localization conditions of the involved systems of α-molecules are satisfied. We finally utilize these results to enable the derivation of optimal sparse approximation results for a specific class of cartoon-like functions by sufficient conditions on the 'control' parameters of a system of α-molecules.

Keywords : Anisotropic Scaling, Curvelets, Nonlinear Approximation, Ridgelets, Shearlets, Sparsity Equivalence, Wavelets

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