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数学物理学报(英文版) ›› 2024, Vol. 44 ›› Issue (3): 823-838.doi: 10.1007/s10473-024-0303-z

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ON A UNIVERSAL INEQUALITY FOR APPROXIMATE PHASE ISOMETRIES

Duanxu Dai1, Haixin Que1, Longfa Sun2,*, Bentuo Zheng3   

  1. 1. School of Science, Jimei University, Xiamen 361021, China;
    2. Hebei Key Laboratory of Physics and Energy Technology, School of Mathematics and Physics, North China Electric Power University, Baoding 071003, China;
    3. Department of Mathematical Sciences, University of Memphis, Memphis, TN 38152, USA
  • 收稿日期:2022-10-26 修回日期:2023-04-15 出版日期:2024-06-25 发布日期:2024-05-21

ON A UNIVERSAL INEQUALITY FOR APPROXIMATE PHASE ISOMETRIES

Duanxu Dai1, Haixin Que1, Longfa Sun2,*, Bentuo Zheng3   

  1. 1. School of Science, Jimei University, Xiamen 361021, China;
    2. Hebei Key Laboratory of Physics and Energy Technology, School of Mathematics and Physics, North China Electric Power University, Baoding 071003, China;
    3. Department of Mathematical Sciences, University of Memphis, Memphis, TN 38152, USA
  • Received:2022-10-26 Revised:2023-04-15 Online:2024-06-25 Published:2024-05-21
  • Contact: *Longfa Sun, E-mail:sun.longfa@163.com
  • About author:Duanxu Dai,E-mail:dduanxu@163.com; Haixin Que, 1131871505@qq.com
  • Supported by:
    Dai's research was supported by the NSFC (12126329, 12171266, 12126346), the NSF of Fujian Province of China (2023J01805) and the Research Start-Up Fund of Jimei University (ZQ2021017); Sun's research was supported by the NSFC (12101234), the NSF of Hebei Province (A2022502010), the Fundamental Research Funds for the Central Universities (2023MS164) and the China Scholarship Council; Zheng's research was supported by the Simons Foundation (585081).

摘要: Let X and Y be two normed spaces. Let U be a non-principal ultrafilter on N. Let g:XY be a standard ε-phase isometry for some ε0, i.e., g(0)=0, and for all u,vX,
||g(u)+g(v)±g(u)g(v)||u+v±uv||ε.
The mapping g is said to be a phase isometry provided that ε=0.
In this paper, we show the following universal inequality of g: for each uw-exp uBX, there exist a phase function σu:X{1,1} and φ Y with φ=uα satisfying that
|u,uσu(u)φ,g(u)|52εα,foralluX. In particular, let X be a smooth Banach space. Then we show the following:
(1) the universal inequality holds for all uX;
(2) the constant 52 can be reduced to 32 provided that Y is strictly convex;
(3) the existence of such a g implies the existence of a phase isometry Θ:XY such that Θ(u)=lim provided that Y^{**} has the w^*-Kadec-Klee property (for example, Y is both reflexive and locally uniformly convex).

关键词: \varepsilon-phase isometry, phase isometry, Banach space

Abstract: Let X and Y be two normed spaces. Let \mathcal{U} be a non-principal ultrafilter on \mathbb{N}. Let g: X\rightarrow Y be a standard \varepsilon-phase isometry for some \varepsilon\geq 0, i.e., g(0)=0, and for all u,v\in X,
|\; |\|g(u)+g(v)\|\pm \|g(u)-g(v)\||-|\|u+v\|\pm\|u-v\||\;|\leq\varepsilon.
The mapping g is said to be a phase isometry provided that \varepsilon=0.
In this paper, we show the following universal inequality of g: for each u^*\in w^*-exp \|u^*\|B_{X^*}, there exist a phase function \sigma_{u^*}: X\rightarrow \{-1,1\} and \varphi \in Y^* with \|\varphi\|= \|u^*\|\equiv \alpha satisfying that
\;\;\;\;\; |\langle u^*,u\rangle-\sigma_{u^*} (u)\langle \varphi, g(u)\rangle |\leq\frac{5}{2}\varepsilon\alpha ,\;\;{\rm for\;all\;}u\in X. In particular, let X be a smooth Banach space. Then we show the following:
(1) the universal inequality holds for all u^*\in X^*;
(2) the constant \frac{5}{2} can be reduced to \frac{3}{2} provided that Y^\ast is strictly convex;
(3) the existence of such a g implies the existence of a phase isometry \Theta:X\rightarrow Y such that \Theta(u)=\lim\limits_{n,\mathcal{U}}\frac{g(nu)}{n} provided that Y^{**} has the w^*-Kadec-Klee property (for example, Y is both reflexive and locally uniformly convex).

Key words: \varepsilon-phase isometry, phase isometry, Banach space

中图分类号: 

  • 46B04