Mathematics – Functional Analysis
Scientific paper
2007-06-11
Proc. Amer. Math. Soc., 136:3013-3023. 2008
Mathematics
Functional Analysis
10 pages. To appear in Proceedings of the American Mathematical Society
Scientific paper
10.1090/S0002-9939-08-09536-1
Results on matrix canonical forms are used to give a complete description of the higher rank numerical range of matrices arising from the study of quantum error correction. It is shown that the set can be obtained as the intersection of closed half planes (of complex numbers). As a result, it is always a convex set in $\mathcal C$. Moreover, the higher rank numerical range of a normal matrix is a convex polygon determined by the eigenvalues. These two consequences confirm the conjectures of Choi et al. on the subject. In addition, the results are used to derive a formula for the optimal upper bound for the dimension of a totally isotropic subspace of a square matrix, and verify the solvability of certain matrix equations.
Li Chi-Kwong
Sze Nung-Sing
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