Rational Approximation, Oscillatory Cauchy Integrals, and Fourier Transforms

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 derived 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 (Numer Math 36(2):197–209, 1980) and Weideman (Math Comput 64(210):745–745, 1995) in a modified basis used by Olver (2011) that behaves well with respect to function multiplication and differentiation.