Numerical radius and distance from unitary operators
نویسندگان
چکیده
منابع مشابه
Numerical radius and distance from unitary operators
Denote by w(A) the numerical radius of a bounded linear operator A acting on Hilbert space. Suppose that A is invertible and that w(A) ≤ 1+ε and w(A−1) ≤ 1+ε for some ε ≥ 0. It is shown that inf{‖A−U‖ : U unitary} ≤ cε for some constant c > 0. This generalizes a result due to J.G. Stampfli, which is obtained for ε = 0. An example is given showing that the exponent 1/4 is optimal. The more gener...
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ژورنال
عنوان ژورنال: Operators and Matrices
سال: 2013
ISSN: 1846-3886
DOI: 10.7153/oam-07-16