Almost preserved extreme points
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arXiv
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| Main Authors: | , , , , |
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| Format: | Preprint |
| Published: |
2025
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| _version_ | 1866915573881896960 |
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| author | Aliaga, Ramón J. García-Lirola, Luis C. Guerrero-Viu, Juan Raja, Matías Zoca, Abraham Rueda |
| author_facet | Aliaga, Ramón J. García-Lirola, Luis C. Guerrero-Viu, Juan Raja, Matías Zoca, Abraham Rueda |
| contents | In this paper we introduce the notion of an almost preserved extreme point (APEP) of a set as a weakening of the concept of preserved extreme points, and we systematically study such points. As a main result, we prove that a Banach space $X$ has the Radon-Nikodým property (RNP) if and only if every closed, convex, and bounded subset of the space has an APEP. Similarly, we prove that $X$ has the RNP if and only if the unit ball of every equivalent renorming has an APEP. We further investigate APEPs of the unit ball of classical Banach spaces, absolute sums, Lipschitz-free spaces, and projective tensor products. In the latter setting, our work also describes the preserved extreme points in the unit ball under the assumption that every bounded operator is compact, thereby partially solving an open problem. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2510_21234 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Almost preserved extreme points Aliaga, Ramón J. García-Lirola, Luis C. Guerrero-Viu, Juan Raja, Matías Zoca, Abraham Rueda Functional Analysis 46B20 (Primary) 46B22, 46B28 (Secondary) In this paper we introduce the notion of an almost preserved extreme point (APEP) of a set as a weakening of the concept of preserved extreme points, and we systematically study such points. As a main result, we prove that a Banach space $X$ has the Radon-Nikodým property (RNP) if and only if every closed, convex, and bounded subset of the space has an APEP. Similarly, we prove that $X$ has the RNP if and only if the unit ball of every equivalent renorming has an APEP. We further investigate APEPs of the unit ball of classical Banach spaces, absolute sums, Lipschitz-free spaces, and projective tensor products. In the latter setting, our work also describes the preserved extreme points in the unit ball under the assumption that every bounded operator is compact, thereby partially solving an open problem. |
| title | Almost preserved extreme points |
| topic | Functional Analysis 46B20 (Primary) 46B22, 46B28 (Secondary) |
| url | https://arxiv.org/abs/2510.21234 |