The algebraic numerical range as a spectral set in Banach algebras
Hanna Blazhko, Daniil Homza, Felix L. Schwenninger, Jens de Vries, Michał Wojtylak · Canadian Journal of Mathematics · 2025
Abstract We investigate when the algebraic numerical range is a C-spectral set in a Banach algebra. While providing several counterexamples based on classical ideas as well as combinatorial Banach spaces, we discuss positive results for matrix algebras and provide an absolute constant in the case of complex $2\times 2$ -matrices with the induced $1$ -norm. Furthermore, we discuss positive results for infinite-dimensional Banach algebras, including the Calkin algebra.