Noncompactness principles in nonlinear operator approximation theory

External approximation of nonlinear operator equations

Based upon an external approximation scheme for the underlying Banach space, a nonlinear operator equation is approximated by a sequence of coercive problems. The equation is supposed to be governed by the sum of two nonlinear operators acting between a reflexive Banach space and its dual. Under suitable stability assumptions and if the underlying operators can be approximated consistently, wea...

Nonlinear Approximation and Adaptive Techniques for Solving Elliptic Operator Equations

This survey article is concerned with two basic approximation concepts and their interrelation with the numerical solution of elliptic operator equations, namely nonlinear and adaptive approximation. On one hand, for nonlinear approximation based on wavelet expansions the best possible approximation rate, which a function can have for a given number of degrees of freedom, is characterized in te...

Nonlinear piecewise polynomial approximation: Theory and Algorithms

Nonlinear piecewise polynomial approximation: Theory and Algorithms Borislav Karaivanov We study nonlinear n-term approximation in Lp(R) (0 < p < ∞) from Courant elements or (discontinuous) piecewise polynomials generated by multilevel nested triangulations of R which allow arbitrarily sharp angles. To characterize the rate of approximation we introduce and develop three families of smoothness ...

Nonlinear approximation theory on finite groups

Motivated by problems in signal recovery, we will investigate the distribution of the energy of the Fourier transform of a positive function on a finite group. In particular, we are able to bound from below the fraction of energy contained in various subsets of the Fourier transform of a positive function defined on a finite group. Applications to signal recovery for positive functions, as well...

A Pragmati Approximation Operator