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BANACH SPACES IN WHICH LARGE SUBSETS OF SPHERES CONCENTRATE

Part of: Set theory Linear function spaces and their duals Normed linear spaces and Banach spaces; Banach lattices Fairly general properties

Published online by Cambridge University Press:  27 December 2022

Piotr Koszmider Open the ORCID record for Piotr Koszmider [Opens in a new window]
Piotr Koszmider*
Affiliation:
Institute of Mathematics of the Polish Academy of Sciences, ul. Śniadeckich 8, 00-656 Warszawa, Poland
*
(piotr.math@proton.me)
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Abstract

We construct a nonseparable Banach space $\mathcal {X}$ (actually, of density continuum) such that any uncountable subset $\mathcal {Y}$ of the unit sphere of $\mathcal {X}$ contains uncountably many points distant by less than $1$ (in fact, by less then $1-\varepsilon $ for some $\varepsilon>0$). This solves in the negative the central problem of the search for a nonseparable version of Kottman’s theorem which so far has produced many deep positive results for special classes of Banach spaces and has related the global properties of the spaces to the distances between points of uncountable subsets of the unit sphere. The property of our space is strong enough to imply that it contains neither an uncountable Auerbach system nor an uncountable equilateral set. The space is a strictly convex renorming of the Johnson–Lindenstrauss space induced by an $\mathbb {R}$-embeddable almost disjoint family of subsets of $\mathbb {N}$. We also show that this special feature of the almost disjoint family is essential to obtain the above properties.

MSC classification

Primary: 46B20: Geometry and structure of normed linear spaces 03E75: Applications of set theory 46B26: Nonseparable Banach spaces
Secondary: 46E15: Banach spaces of continuous, differentiable or analytic functions 54D80: Special constructions of spaces (spaces of ultrafilters, etc.)
Type
Research Article
Information
Journal of the Institute of Mathematics of Jussieu , Volume 23 , Issue 2 , March 2024 , pp. 737 - 752
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Copyright
© The Author(s), 2022. Published by Cambridge University Press

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