Non-commutative Schur-Horn theorems and extended majorization for Hermitian matrices

Abstract

Let A ⊆ Mn(C) be a unital ∗-subalgebra of the algebra Mn(C) of all n×n complex matrices and let B be an hermitian matrix. Let Un(B) denote the unitary orbit of B in Mn(C) and let EA denote the trace preserving conditional expectation onto A. We give a spectral characterization of the set EA(Un(B)) = {EA(U ∗B U) : U ∈ Mn(C), unitary matrix}. We obtain a similar result for the contractive orbit of a positive semi-definite matrix B. We then use these results to extend the notions of majorization and submajorization between self-adjoint matrices to spectral relations that come together with extended (non-commutative) Schur-Horn type theorems.