Multivariable Schur-Horn theorems

Abstract

We prove a variety of results describing the possible diagonals of tuples of commuting hermitian operators in type $I{I}_1$ 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.

31 pages, no figures

References in corpus (3)

• Diagonals of self-adjoint operators
• Diagonals of normal operators with finite spectrum
• The Schur-Horn theorem in von Neumann algebras

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• Closed convex hulls of unitary orbits in certain simple real rank zero C${}^{\ast }$-algebras
• Submajorization on ${\ell }^p(I{)}^{+}$ determined by increasable doubly substochastic operators and its linear preservers
• Diagonals of operators and Blaschke's enigma
• On interplay between operators, bases, and matrices
• Extreme points of the set of elements majorised by an integrable function: Resolution of a problem by Luxemburg and of its noncommutative counterpart