On the boundary of numerical ranges of operators
نویسندگان
چکیده
منابع مشابه
On Generalized Numerical Ranges of Quadratic Operators
It is shown that the result of Tso-Wu on the elliptical shape of the numerical range of quadratic operators holds also for the essential numerical range. The latter is described quantitatively, and based on that sufficient conditions are established under which the c-numerical range also is an ellipse. Several examples are considered, including singular integral operators with the Cauchy kernel...
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Composition operators on the Hilbert Hardy space of the unit disk are considered. The shape of their numerical range is determined in the case when the symbol of the composition operator is a monomial or an inner function fixing 0. Several results on the numerical range of composition operators of arbitrary symbol are obtained. It is proved that 1 is an extreme boundary point if and only if 0 i...
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ژورنال
عنوان ژورنال: Applied Mathematics Letters
سال: 2011
ISSN: 0893-9659
DOI: 10.1016/j.aml.2010.11.024