{"title":"Almost Auerbach, Markushevich and Schauder bases in Hilbert and Banach spaces","authors":"Anton Tselishchev","doi":"10.1016/j.jfa.2025.110895","DOIUrl":null,"url":null,"abstract":"<div><div>For any sequence of positive numbers <span><math><msubsup><mrow><mo>(</mo><msub><mrow><mi>ε</mi></mrow><mrow><mi>n</mi></mrow></msub><mo>)</mo></mrow><mrow><mi>n</mi><mo>=</mo><mn>1</mn></mrow><mrow><mo>∞</mo></mrow></msubsup></math></span> such that <span><math><msubsup><mrow><mo>∑</mo></mrow><mrow><mi>n</mi><mo>=</mo><mn>1</mn></mrow><mrow><mo>∞</mo></mrow></msubsup><msub><mrow><mi>ε</mi></mrow><mrow><mi>n</mi></mrow></msub><mo>=</mo><mo>∞</mo></math></span> we provide an explicit simple construction of <span><math><mo>(</mo><mn>1</mn><mo>+</mo><msub><mrow><mi>ε</mi></mrow><mrow><mi>n</mi></mrow></msub><mo>)</mo></math></span>-bounded Markushevich basis in a separable Hilbert space which is not strong, or, in other terminology, is not hereditary complete; this condition on the sequence <span><math><msubsup><mrow><mo>(</mo><msub><mrow><mi>ε</mi></mrow><mrow><mi>n</mi></mrow></msub><mo>)</mo></mrow><mrow><mi>n</mi><mo>=</mo><mn>1</mn></mrow><mrow><mo>∞</mo></mrow></msubsup></math></span> is sharp. Using a finite-dimensional version of this construction, Dvoretzky's theorem and a construction of Vershynin, we conclude that in any Banach space for any sequence of positive numbers <span><math><msubsup><mrow><mo>(</mo><msub><mrow><mi>ε</mi></mrow><mrow><mi>n</mi></mrow></msub><mo>)</mo></mrow><mrow><mi>n</mi><mo>=</mo><mn>1</mn></mrow><mrow><mo>∞</mo></mrow></msubsup></math></span> such that <span><math><msubsup><mrow><mo>∑</mo></mrow><mrow><mi>n</mi><mo>=</mo><mn>1</mn></mrow><mrow><mo>∞</mo></mrow></msubsup><msubsup><mrow><mi>ε</mi></mrow><mrow><mi>n</mi></mrow><mrow><mn>2</mn></mrow></msubsup><mo>=</mo><mo>∞</mo></math></span> there exists a <span><math><mo>(</mo><mn>1</mn><mo>+</mo><msub><mrow><mi>ε</mi></mrow><mrow><mi>n</mi></mrow></msub><mo>)</mo></math></span>-bounded Markushevich basis which is not a Schauder basis after any permutation of its elements.</div></div>","PeriodicalId":15750,"journal":{"name":"Journal of Functional Analysis","volume":"289 1","pages":"Article 110895"},"PeriodicalIF":1.7000,"publicationDate":"2025-03-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"Journal of Functional Analysis","FirstCategoryId":"100","ListUrlMain":"https://www.sciencedirect.com/science/article/pii/S0022123625000771","RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q1","JCRName":"MATHEMATICS","Score":null,"Total":0}
引用次数: 0
Abstract
For any sequence of positive numbers such that we provide an explicit simple construction of -bounded Markushevich basis in a separable Hilbert space which is not strong, or, in other terminology, is not hereditary complete; this condition on the sequence is sharp. Using a finite-dimensional version of this construction, Dvoretzky's theorem and a construction of Vershynin, we conclude that in any Banach space for any sequence of positive numbers such that there exists a -bounded Markushevich basis which is not a Schauder basis after any permutation of its elements.
期刊介绍:
The Journal of Functional Analysis presents original research papers in all scientific disciplines in which modern functional analysis plays a basic role. Articles by scientists in a variety of interdisciplinary areas are published.
Research Areas Include:
• Significant applications of functional analysis, including those to other areas of mathematics
• New developments in functional analysis
• Contributions to important problems in and challenges to functional analysis