Rational approximation, oscillatory Cauchy integrals and Fourier transforms

TL;DR

This paper develops convergence theory and error estimates for rational function interpolation on the real axis, with applications to oscillatory Fourier transforms and Cauchy integrals, extending previous methods with improved bounds.

Contribution

It introduces new error bounds for rational interpolation and applies these to analyze oscillatory Fourier transforms and Cauchy integrals, extending existing theoretical frameworks.

Findings

Derived new error estimates for rational interpolants.

Established bounds for Fourier transform and Cauchy integral of oscillatory functions.

Analyzed the behavior of the differentiation operator in this context.

Abstract

We develop the convergence theory for a well-known method for the interpolation of functions on the real axis with rational functions. Precise new error estimates for the interpolant are de- rived using existing theory for trigonometric interpolants. Estimates on the Dirichlet kernel are used to derive new bounds on the associated interpolation projection operator. Error estimates are desired partially due to a recent formula of the author for the Cauchy integral of a specific class of so-called oscillatory rational functions. Thus, error bounds for the approximation of the Fourier transform and Cauchy integral of oscillatory smooth functions are determined. Finally, the behavior of the differentiation operator is discussed. The analysis here can be seen as an extension of that of Weber (1980) and Weideman (1995) in a modified basis used by Olver (2009) that behaves well with respect to function multiplication and differentiation.