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Displaying similar documents to “Method of orthogonal projections and approximation of the spectrum of a bounded operator II”

On an estimate for the norm of a function of a quasihermitian operator

M. Gil (1992)

Studia Mathematica

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Let A be a closed linear operator acting in a separable Hilbert space. Denote by co(A) the closed convex hull of the spectrum of A. An estimate for the norm of f(A) is obtained under the following conditions: f is a holomorphic function in a neighbourhood of co(A), and for some integer p the operator A p - ( A * ) p is Hilbert-Schmidt. The estimate improves one by I. Gelfand and G. Shilov.