Numerical Ranges and Spectral Sets: the unbounded case

Published 24 Sep 2025 in math.FA, cs.NA, and math.NA | (2509.19792v1)

Abstract: It is known that, if $\Omega$ $\subset$ C is a convex set containing the numerical range of an operator A, then $\Omega$ is a C $\Omega$ -spectral set for A with C $\Omega$ $\le$ 1+ $\sqrt$ 2. We improve this estimate in unbounded cases.

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Michel Crouzeix

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