Bourbaki

Exercise 18, § 2

Tag 0275, permanent. Cite it as [Bourbaki, Tag 0275].

Spectral Theories, I, p. 159

Let A be a unital Banach algebra. The set of the xx which are neither invertible nor topological zero divisors, is open in A.

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Machine translation of the English, by gpt-5-mini. Not checked by a person. That model is a cut down version of the one the rest of this language was written by.