Generalised differences and a class of multiplier operators in Fourier analysis

Abstract

The ranges of a certain type of second order differential operator, on a Sobolev subspace of the Lebesgue space $L^2$ of the circle group, can be characterised by the vanishing of the Fourier coefficients at (generally) two integers that are the zeros of the multiplier of the operator. It is proved here that the range of any such operator may be alternatively described as comprising those functions in $L^2$ that are the sum of five generalised second order differences, each such difference involving the zeros of the multiplier. In fact, higher order operators and differences are considered. There are applications to automatic continuity of linear forms on $L^2$. This work is related to earlier work of G. Meisters and W. Schmidt who derived, in effect, a description of the range of the ordinary differentiation operator D (whose multiplier vanishes at 0) in terms of first order differences.