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Let be an operator acting on a Banach space . We show that between extensions of to some Banach space which do not increase the defect spectrum (or the spectrum) it is possible to find an extension with the minimal possible defect spectrum.
An operator in a Banach space is called upper (lower) semi-Browder if it is upper (lower) semi-Fredholm and has a finite ascent (descent). We extend this notion to n-tuples of commuting operators and show that this notion defines a joint spectrum. Further we study relations between semi-Browder and (essentially) semiregular operators.
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