Abstract
We investigate two systematic constructions of inverse-closed subalgebras of a given Banach algebra or operator algebra \(\ensuremath {\mathcal {A}}\), both of which are inspired by classical principles of approximation theory. The first construction requires a closed derivation or a commutative automorphism group on \(\ensuremath {\mathcal {A}}\) and yields a family of smooth inverse-closed subalgebras of \(\ensuremath {\mathcal {A}}\) that resemble the usual Hölder–Zygmund spaces. The second construction starts with a graded sequence of subspaces of \(\ensuremath{\mathcal{A}}\) and yields a class of inverse-closed subalgebras that resemble the classical approximation spaces. We prove a theorem of Jackson–Bernstein type to show that in certain cases both constructions are equivalent.
These results about abstract Banach algebras are applied to algebras of infinite matrices with off-diagonal decay. In particular, we obtain new and unexpected conditions of off-diagonal decay that are preserved under matrix inversion.
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Communicated by Albert Cohen.
K.G. was supported by the Marie-Curie Excellence Grant MEXT-CT 2004-517154. A.K. was supported by the grant MA44 MOHAWI of the Vienna Science and Technology Fund (WWTF) and partially by the National Research Network S106 SISE of the Austrian Science Foundation (FWF).
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Gröchenig, K., Klotz, A. Noncommutative Approximation: Inverse-Closed Subalgebras and Off-Diagonal Decay of Matrices. Constr Approx 32, 429–466 (2010). https://doi.org/10.1007/s00365-010-9101-z
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DOI: https://doi.org/10.1007/s00365-010-9101-z
Keywords
- Banach algebra
- Matrix algebra
- Smoothness space
- Inverse closedness
- Spectral invariance
- Offf-diagonal decay
- Automorphism group
- Jackson–Bernstein theorem