Geometric means of quasi-Toeplitz matrices

TL;DR

This paper investigates geometric means of quasi-Toeplitz matrices, showing they are closely related to the geometric mean of their generating functions, with practical approximation methods demonstrated.

Contribution

It establishes that geometric means of quasi-Toeplitz matrices correspond to the geometric mean of their symbols, differing only by compact operators, extending classical Toeplitz results.

Findings

Geometric means of quasi-Toeplitz matrices are quasi-Toeplitz with the geometric mean of symbols.

Theoretical results apply to continuous and Wiener algebra functions.

Numerical tests confirm practical approximation of these operator means.

Abstract

We study means of geometric type of quasi-Toeplitz matrices, that are semi-infinite matrices $A=(a_{i,j})_{i,j=1,2,\ldots}$ of the form $A=T(a)+E$, where $E$ represents a compact operator, and $T(a)$ is a semi-infinite Toeplitz matrix associated with the function $a$, with Fourier series $\sum_{\ell=-\infty}^{\infty} a_\ell e^{\mathfrak i \ell t}$, in the sense that $(T(a))_{i,j}=a_{j-i}$. If $a$ is \rv\ and essentially bounded, then these matrices represent bounded self-adjoint operators on $\ell^2$. We consider the case where $a$ is a continuous function, where quasi-Toeplitz matrices coincide with a classical Toeplitz algebra, and the case where $a$ is in the Wiener algebra, that is, has absolutely convergent Fourier series. We prove that if $a_1,\ldots,a_p$ are continuous and positive functions, or are in the Wiener algebra with some further conditions, then means of geometric type, such as the ALM, the NBMP and the Karcher mean of quasi-Toeplitz positive definite matrices associated with $a_1,\ldots,a_p$, are quasi-Toeplitz matrices associated with the geometric mean $(a_1\cdots a_p)^{1/p}$, which differ only by the compact correction. We show by numerical tests that these operator means can be practically approximated.