Localized Summability Kernels for Jacobi Expansions

Abstract

While the direct and converse theorems of approximation theory enable us to characterize the smoothness of a function f:[−1,1] → R in terms of its degree of polynomial approximation, they do not account for local smoothness. The use of localized summability kernels leads to a wavelet-like representation, using the Fourier–Jacobi coefficients of f, so as to characterize the smoothness of f in a neighborhood of each point in terms of the behavior of the terms of this representation. In this paper, we study the localization properties of a class of kernels, which have explicit forms in the "space domain," and establish explicit bounds on the Lebesgue constants on the summability kernels corresponding to some of these.

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