The multiplicative Hilbert matrix

TL;DR

This paper introduces the multiplicative Hilbert matrix, analyzes its spectral properties, and compares it to the classical Hilbert matrix, revealing its norm, spectrum, and boundedness characteristics on various function spaces.

Contribution

It defines a new multiplicative Hilbert matrix, establishes its norm and spectral properties, and explores its boundedness on different Dirichlet and Hardy spaces, connecting it to classical results.

Findings

The norm of the multiplicative Hilbert matrix is π.

It has a purely continuous spectrum equal to [0, π].

The matrix is unbounded on certain Hardy spaces for p ≠ 2.

Abstract

It is observed that the infinite matrix with entries $(\sqrt{mn}\log (mn))^{-1}$ for $m, n\ge 2$ appears as the matrix of the integral operator $\mathbf{H}f(s):=\int_{1/2}^{+\infty}f(w)(\zeta(w+s)-1)dw$ with respect to the basis $(n^{-s})_{n\ge 2}$; here $\zeta(s)$ is the Riemann zeta function and $H$ is defined on the Hilbert space ${\mathcal H}^2_0$ of Dirichlet series vanishing at $+\infty$ and with square-summable coefficients. This infinite matrix defines a multiplicative Hankel operator according to Helson's terminology or, alternatively, it can be viewed as a bona fide (small) Hankel operator on the infinite-dimensional torus ${\Bbb T}^{\infty}$. By analogy with the standard integral representation of the classical Hilbert matrix, this matrix is referred to as the multiplicative Hilbert matrix. It is shown that its norm equals $\pi$ and that it has a purely continuous spectrum which is the interval $[0,\pi]$; these results are in agreement with known facts about the classical Hilbert matrix. It is shown that the matrix $(m^{1/p} n^{(p-1)/p}\log (mn))^{-1}$ has norm $\pi/\sin(\pi /p)$ when acting on $\ell^p$ for $1<p<\infty$. However, the multiplicative Hilbert matrix fails to define a bounded operator on ${\mathcal H}^p_0$ for $p\neq 2$, where ${\mathcal H}^p_0$ are $H^p$ spaces of Dirichlet series. It remains an interesting problem to decide whether the analytic symbol $\sum_{n\ge 2} (\log n)^{-1} n^{-s-1/2}$ of the multiplicative Hilbert matrix arises as the Riesz projection of a bounded function on the infinite-dimensional torus ${\Bbb T}^\infty$.