Abstract
Let F be a closed subset of X = M(L∞) and let a ∈ L ∞ N×N . The matrix function a is called analytically sectorial on F if there exist a real number ε > 0 and two invertible matrices b, c ∈ ℂ N×N such that Re (ba(x) c) ≧ ε for all x ∈ F, that is,
and a is said to be geometrically sectorial on F if
that is, if each matrix in the closed convex hull of a(F) is invertible.
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Böttcher, A., and B. Silbermann, Local spectra of approximate identities, cluster sets, and Toeplitz operators. Wiss. Z. Tech. Hochsch. Karl-Marx-Stadt 28: 2, 175–180 (1986).
Sarason, D., Toeplitz operators with piecewise quasicontinuous symbols. Indiana Univ. Math. J. 26: 5, 817–838 (1977).
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Böttcher, A., Silbermann, B. (1990). Symbol analysis. In: Analysis of Toeplitz Operators. Springer Monographs in Mathematics. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-662-02652-6_3
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