The Fourier, Hilbert and Mellin transforms on a half-line

Abstract

We are interested in the singular behaviour at the origin of solutions to the equation $\mathscr{H} \rho = e$ on a half-axis, where $\mathscr H$ is the one-sided Hilbert transform, $\rho$ an unknown solution and $e$ a known function. This is a simpler model problem on the path to understanding wave field singularities caused by curve-shaped scatterers in a planar domain. We prove that $\rho$ has a singularity of the form $\mathscr M[e](1/2) / \sqrt{t}$ where $\mathscr M$ is the Mellin transform. To do this we use specially built function spaces $\mathscr M'(a,b)$ by Zemanian, and these allow us to precisely investigate the relationship between the Mellin and Hilbert transforms. Fourier comes into play in the sense that the Mellin transform is simpy the Fourier transform on the locally compact Abelian multiplicative group of the half-line, and as a more familiar operator it guides our investigation.