Abstract
In 1989, G. Godefroy proved that a Banach space contains an isomorphic copy of ℓ1 if and only if it can be equivalently renormed to be octahedral. It is known that octahedral norms can be characterized by means of covering the unit sphere by a finite number of balls. This observation allows us to connect the theory of octahedral norms with ball-covering properties of Banach spaces introduced by L. Cheng in 2006. Following this idea, we extend G. Godefroy’s result to higher cardinalities. We prove that, for a regular cardinal κ, a Banach space X contains an isomorphic copy of ℓ1(κ) if and only if it can be equivalently renormed in such a way that its unit sphere cannot be covered by strictly less than κ many open balls not containing αBX, where α ∊ (0, 1). We also investigate the relation between ball-coverings of the unit sphere and octahedral norms in the setting of higher cardinalities.
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Acknowledgements
The authors would like to thank the anonymous referee for a careful reading of the manuscript and for suggestions that improved the exposition, in particular, for pointing out a better statement of Theorem 3.1.
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This work was supported by the Estonian Research Council grants (PSG487) and (PRG877).
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Ciaci, S., Langemets, J. & Lissitsin, A. A characterization of Banach spaces containing ℓ1(κ) via ball-covering properties. Isr. J. Math. 253, 359–379 (2023). https://doi.org/10.1007/s11856-022-2363-x
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DOI: https://doi.org/10.1007/s11856-022-2363-x