On Abstract Spectral Constants

02/10/2023

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by   Felix L. Schwenninger, et al.

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We prove bounds for a class of unital homomorphisms arising in the study of spectral sets, by involving extremal functions and vectors. These are used to recover three celebrated results on spectral constants by Crouzeix–Palencia, Okubo–Ando and von Neumann in a unified way and to refine a recent result by Crouzeix–Greenbaum.