Multivariable Schur-Horn theorems

TL;DR

This paper extends the classical Schur-Horn theorem to multivariable settings in type II_1 factors, providing a comprehensive description of possible diagonals of commuting hermitian operators using a generalized majorization concept.

Contribution

It introduces a multivariable generalization of the Schur-Horn theorem for infinite-dimensional operators, including complete results for finite joint spectra and approximate results for infinite spectra.

Findings

Complete characterization for finite joint spectrum cases

Strong approximate results for infinite joint spectrum

Identification of obstructions in extending finite spectrum results

Abstract

We prove a variety of results describing the possible diagonals of tuples of commuting hermitian operators in type $II_1$ factors. These results are generalisations of the classical Schur-Horn theorem to the infinite dimensional, multivariable setting. Our description of these possible diagonals uses a natural generalisation of the classical notion of majorization to the multivariable setting. In the special case when both the given tuple and the desired diagonal have finite joint spectrum, our results are complete. When the tuples do not have finite joint spectrum, we are able to prove strong approximate results. Unlike the single variable case, the multivariable case presents several surprises and we point out obstructions to extending our complete description in the finite spectrum case to the general case. We also discuss the problem of characterizing diagonals of commuting tuples in $\mathcal{B}(\mathcal{H})$ and give approximate characterizations in this case as well.