The Schur-Horn theorem for operators with finite spectrum

Details The Schur-Horn theorem for operators with finite spectrum The Schur-Horn theorem for operators with finite spectrum

2011-11-16

arxiv.org/abs/1111.3833v1

Mathematics

Operator Algebras

10 pages, no figures

Scientific paper

The carpenter problem in the context of $II_1$ factors, formulated by Kadison asks: Let $\mathcal{A} \subset \mathcal{M}$ be a masa in a type $II_1$ factor and let $E$ be the normal conditional expectation from $\mathcal{M}$ onto $\mathcal{A}$. Then, is it true that for every positive contraction $A$ in $\mathcal{A}$, there is a projection $P$ in $\mathcal{M}$ such that $E(P) = A$? In this note, we show that this is true if $A$ has finite spectrum. We will then use this result to prove an exact Schur-Horn theorem for (positive)operators with finite spectrum and an approximate Schur-Horn theorem for general (positive)operators.

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