English

On the definition of "almost LUR (ALUR)" notion

Functional Analysis 2025-02-25 v2

Abstract

The notion of almost LUR (ALUR) point is introduced in the paper [P. Bandyopadhyay et al., Some generalizations of locally uniform rotundity, J. Math. Anal. Appl., 252, 906-916 (2000)], where one says that the point xx of the unit sphere SXS_{X} of a Banach space is an almost LUR (ALUR) point of BXB_{X} if for any sequences {xn}BX\{x_{n}\}\subseteq B_{X} and {xm}BX\{x_{m}^{\ast}\}\subseteq B_{X^{\ast}}, the condition limmlimnxm(xn+x2)=1\lim_{m}\lim_{n}x_{m}^{\ast}\left(\frac{x_{n}+x}{2}\right)=1 implies limnxnx=0\lim_{n}\left\Vert x_{n}-x\right\Vert =0, without mentioning what is meant by limmlimnγm,n=γ\lim_{m}\lim_{n}\gamma_{m,n}=\gamma for γ\gamma, γm,nR\gamma_{m,n}\in\mathbb{R}; XX is ALUR if XX is almost LUR at any xSXx\in S_{X}. Of course, the natural definition for this iterated limit would be that for each mm sufficiently large there exists μm:=limnγm,nR\mu_{m}:=\lim_{n\rightarrow\infty}\gamma_{m,n}\in\mathbb{R} and γ=limmμm\gamma =\lim_{m\rightarrow\infty}\mu_{m}. However, as seen in some proofs where limmlimn\lim_{m}\lim_{n} appears, this interpretation is not confirmed. In this paper we examine several works in which almost LUR is mentioned and, especially, the proofs of those results in which the above definition of "almost LUR" point (or space) is invoked. Moreover, we analyze similar problems related to the notion CWALUR which extend ALUR. Furthermore, we mention several gaps in the proofs of some results. Finally, we propose the change of limmlimn\lim_{m}\lim_{n} by limmlim infn\lim_{m}\liminf_{n} in the definitions of several types of ALUR points; moreover, we provide the complete proofs of two results from the literature in which limmlimn\lim_{m}\lim_{n} were used effectively, using limmlim infn\lim_{m}\liminf_{n} instead.

Keywords

Cite

@article{arxiv.2502.11637,
  title  = {On the definition of "almost LUR (ALUR)" notion},
  author = {Constantin Zalinescu},
  journal= {arXiv preprint arXiv:2502.11637},
  year   = {2025}
}

Comments

The paper has 16 pages an a bibliography containing 21 references. In the present version we corrected some inaccuracies and added a subnote on page 10