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Operator norm attainment and Birkhoff-James orthogonality. (English) Zbl 1335.46008

Summary: Recently in [D. Sain and K. Paul, Linear Algebra Appl. 439, No. 8, 2448–2452 (2013; Zbl 1291.46024)] it was proved that if \(T\) is a linear operator on a finite dimensional real normed linear space \(\mathbb{X}\) such that \(T\) attains norm only on \(\pm D\), where \(D\) is a connected closed subset of \(S_{\mathbb{X}}\), then \(T\) satisfies the Bhatia-Šemrl (BŠ) property [R. Bhatia and P. Šemrl, Linear Algebra Appl. 287, No. 1–3, 77–85 (1999; Zbl 0937.15023)], i.e., for \(A \in \mathbb{L}(\mathbb{X})\), \(T \bot_B A\) implies that there exists \(x \in D\) such that \(T x \bot_B A x\). Here we explore the converse of the above result. We prove that in a real normed linear space \(\mathbb{X}\) of dimension \(2\), a linear operator \(T\) satisfies the BŠ property if and only if the set of unit vectors on which \(T\) attains norm is connected in the corresponding projective space \(\mathbb{R} P^1 \equiv S_{\mathbb{X}} / \{x \sim - x \}\). Motivated by the result in 2 dimensions, we conjecture that this characterization of BŠ property is true in general \(n\)-dimensional real normed linear spaces. We further prove that if the space \(\mathbb{X}\) is strictly convex, then the set of operators in \(\mathbb{L}(\mathbb{X})\) which satisfy the BŠ property is dense in \(\mathbb{L}(\mathbb{X})\).

MSC:

46B04 Isometric theory of Banach spaces
46B20 Geometry and structure of normed linear spaces
46C15 Characterizations of Hilbert spaces
47A30 Norms (inequalities, more than one norm, etc.) of linear operators
Full Text: DOI

References:

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