On the defect spectrum of an extension of a Banach space operator
Czechoslovak Mathematical Journal, Tome 48 (1998) no. 4, pp. 609-616.

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Let $T$ be an operator acting on a Banach space $X$. We show that between extensions of $T$ to some Banach space $Y\supset X$ which do not increase the defect spectrum (or the spectrum) it is possible to find an extension with the minimal possible defect spectrum.
Classification : 47A10, 47A20 
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     author = {Kordula, Vladim{\'\i}r},
     title = {On the defect spectrum of an extension of a {Banach} space operator},
     journal = {Czechoslovak Mathematical Journal},
     pages = {609--616},
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Kordula, Vladimír. On the defect spectrum of an extension of a Banach space operator. Czechoslovak Mathematical Journal, Tome 48 (1998) no. 4, pp. 609-616. https://geodesic-test.mathdoc.fr/item/CMJ_1998__48_4_a0/