Unconditional bases and strictly convex dual renormings

关于Banach空间条件基对称类型的注记

Remarks about symmetry-type conditions of conditional bases of Banach spaces

该研究探讨了准Banach空间中条件基的对称性和扩散性，通过重范数构造使排列和递增映射成为等距映射，扩展了经典等距重范数定理的应用范围。

无条件与对称序列空间的$2$-圆范数

$2$-rotund norms for unconditional and symmetric sequence spaces

该研究证明带无条件基的自反Banach空间有等价1-无条件2R范数且可嵌入带1-对称2R范数的自反空间，还得到对称基空间1-对称2R重赋范的部分结果。

Light groups of isomorphisms of Banach spaces and invariant LUR renormings

Megrelishvili defines \emph{light groups} of isomorphisms of a Banach space as the groups on which the Weak and Strong Operator Topologies coincide, and proves that every bounded group of isomorphisms of Banach spaces with the Point of Continuity Property (PCP) is light. We investigate this concept for isomorphism groups $G$ of classical Banach spaces $X$ without the PCP, specially isometry groups, and relate it to the existence of $G$-invariant LUR or strictly convex renormings of $X$.

范数可达格同态与巴拿赫格的重赋范

Norm-attaining lattice homomorphisms and renormings of Banach lattices

本文研究巴拿赫格上格同态的范数可达性，证明其在格同构下不具不变性，刻画原子坐标泛函并构造反例否定相关问题，揭示重赋范对该性质的影响。

On an isomorphic Banach-Mazur rotation problem and maximal norms in Banach spaces

We prove that the spaces $\ell_p$, $1<p<\infty, p\ne 2$, and all infinite-dimensional subspaces of their quotient spaces do not admit equivalent almost transitive renormings. This is a step towards the solution of the Banach-Mazur rotation problem, which asks whether a separable Banach space with a transitive norm has to be isometric or isomorphic to a Hilbert space. We obtain this as a consequence of a new property of almost transitive spaces with a Schauder basis, namely we prove that in such spaces the unit vector basis of $\ell_2^2$ belongs to the two-dimensional asymptotic structure and w…

一个小的 Banach 空间 $C(K)$ 没有好的重赋范

A small Banach space $C(K)$ without nice renormings

在 $\omega_1<\mathfrak{c}$ 的一致假设下，构造了一个紧空间 $K$，使得连续函数空间 $C(K)$ 是 Grothendieck 空间、密度为 $\omega_1$，且不存在严格凸或序列 Kadets–Klee 重赋范。

Isometric embeddings of Banach spaces under optimal projection constants

Let $X$ be a Banach space with separable dual. It is proved that for every $\varepsilon\in (0,1)$, $X$ embeds isometrically into a Banach space $W$ with a shrinking basis $(w_n)$ which is $(1+ \varepsilon)$-monotone. Moreover, if $X$ has further an FDD $(E_n)$ whose strong bimonotonicity projection constant is not larger than $\mathcal{D}$, then $(w_n)$ has strong bimonotonicity projection constant not exceeding $\mathcal{D}(1 +\varepsilon)$. Further, if $(E_n)$ is $\mathcal{C}$-unconditional then $(w_n)$ is $\mathcal{C}(1 + \varepsilon)$-unconditional. The proof uses renorming and skipped blo…

Spaces C(K) with an equivalent URED norm

We prove that a Banach space of continuous functions $C(K)$ has a renorming that is uniformly rotund in every direction (URED) if and only if the compact space $K$ supports a strictly positive measure

Counterexamples in rotundity of norms in Banach spaces

We study several classical concepts in the topic of strict convexity of norms in infinite dimensional Banach spaces. Specifically, and in descending order of strength, we deal with Uniform Rotundity (UR), Weak Uniform Rotundity (WUR) and Uniform Rotundity in Every Direction (URED). Our first three results show that we may distinguish between all of these three properties in every Banach space where such renormings are possible. Specifically, we show that in every infinite dimensional Banach space which admits a WUR (resp. URED) renorming, we can find a norm with the same condition and which mo…

On the embeddability of countably branching bundle graphs into dual spaces

In this note the result by A. Swift concerning the embeddability of countably branching bundle graphs into Banach spaces is extended from the context of reflexive spaces with an unconditional asymptotic structure to the context of dual spaces with a weak$^*$ unconditional asymptotic structure. We also investigate weak$^*$ asymptotic midpoint uniform convexity in dual spaces which turns out to be equivalent to its weak version in general and to the standard weak$^*$ asymptotic uniform convexity up to renorming in dual spaces with a weak$^*$ unconditional asymptotic structure.

Large Banach spaces with no infinite equilateral sets

A subset of a Banach space is called equilateral if the distances between any two of its distinct elements are the same. It is proved that there exist non-separable Banach spaces (in fact of density continuum) with no infinite equilateral subset. These examples are strictly convex renormings of $\ell_1([0,1])$. A wider class of renormings of $\ell_1([0,1])$ which admit no uncountable equilateral sets is also considered.

一个具有无条件基的巴拿赫空间，它不是切线可数确定的

A Banach space with an unconditional basis which is not slicely countably determined

本文研究了一个由无限二叉树分支的适当族生成的巴拿赫空间的单位球几何，并回答了关于切线可数确定巴拿赫空间的相关问题。主要结果是二叉树空间是一个具有无条件基但不切线可数确定的巴拿赫空间，且不包含具有达乌瓦特性质的空间的同构副本。

Steinhaus' lattice-point problem for Banach spaces

Steinhaus proved that given a~positive integer $n$, one may find a circle surrounding exactly $n$ points of the integer lattice. This statement has been recently extended to Hilbert spaces by Zwoleński, who replaced the integer lattice by any infinite set that intersects every ball in at most finitely many points. We investigate Banach spaces satisfying this property, which we call (S), and characterise them by means of a new geometric property of the unit sphere which allows us to show, e.g., that all strictly convex norms have (S), nonetheless, there are plenty of non-strictly convex norms s…

Polyhedrality in Pieces

The aim of this paper is to present two tools, Theorems 4 and 7, that make the task of finding equivalent polyhedral norms on certain Banach spaces easier and more transparent. The hypotheses of both tools are based on countable decompositions, either of the unit sphere S_X or of certain subsets of the dual ball of a given Banach space X. The sufficient conditions of Theorem 4 are shown to be necessary in the separable case. Using Theorem 7, we can unify two known results regarding the polyhedral renorming of certain C(K) spaces, and spaces having an (uncountable) unconditional basis. New exam…

Smooth norms in dense subspaces of Banach spaces

In the first part of our paper, we show that $\ell_\infty$ has a dense linear subspace which admits an equivalent real analytic norm. As a corollary, every separable Banach space, as well as $\ell_1(\mathfrak{c})$, also has a dense linear subspace which admits an analytic renorming. By contrast, no dense subspace of $c_0(ω_1)$ admits an analytic norm. In the second part, we prove (solving in particular an open problem of Guirao, Montesinos, and Zizler) that every Banach space with a long unconditional Schauder basis contains a dense subspace that admits a $C^{\infty}$-smooth norm. Finally, we …

On the fixed point property in direct sums of Banach spaces with strictly monotone norms

It is shown that if a Banach space X has the weak Banach-Saks property and the weak fixed point property for nonexpansive mappings and Y satisfies property asymptotic (P) (which is weaker than the condition WCS(Y)>1), then the direct sum of X and Y endowed with a strictly monotone norm enjoys the weak fixed point property. The same conclusion is valid if X admits a 1-unconditional basis.

Daugavet- and delta-points in Banach spaces with unconditional bases

We study the existence of Daugavet- and delta-points in the unit sphere of Banach spaces with a $1$-unconditional basis. A norm one element $x$ in a Banach space is a Daugavet-point (resp. delta-point) if every element in the unit ball (resp. $x$ itself) is in the closed convex hull of unit ball elements that are almost at distance $2$ from $x$. A Banach space has the Daugavet property (resp. diametral local diameter two property) if and only if every norm one element is a Daugavet-point (resp. delta-point). It is well-known that a Banach space with the Daugavet property does not have an uncon…

1-complemented subspaces of spaces with 1-unconditional bases

We prove that if $X$ is a complex strictly monotone sequence space with $1$-unconditional basis, $Y \subseteq X$ has no bands isometric to $\ell_2^2$ and $Y$ is the range of norm-one projection from $X$, then $Y$ is a closed linear span a family of mutually disjoint vectors in $X$. We completely characterize $1$-complemented subspaces and norm-one projections in complex spaces $\ell_p(\ell_q)$ for $1 \leq p, q < \infty$. Finally we give a full description of the subspaces that are spanned by a family of disjointly supported vectors and which are $1$-complemented in (real or complex) Orlicz or …

The anisotropic fractional isoperimetric problem with respect to unconditional unit balls

The minimizers of the anisotropic fractional isoperimetric inequality with respect to the convex body $K$ in $\mathbb{R}^n$ are shown to be equivalent to star bodies whenever $K$ is strictly convex and unconditional. From this a Pólya-Szegö principle for anisotropic fractional seminorms is derived by using symmetrization with respect to star bodies.

严格凸范数与局部直径二性质

Strictly convex norms and the local diameter two property

本文在实函数实心巴拿赫格中引入范数严格单调性以阻止局部直径二性质，证明$c_0(Γ)$上严格凸1-对称范数具此性质，并构造不可数$Γ$的$c_0(Γ)$上带1-无条件基且具局部直径二性质的严格凸范数。

A class of Banach spaces with few non strictly singular operators

We construct a family $(\mathcal{X}_\al)_{\al\le ω_1}$ of reflexive Banach spaces with long transfinite bases but with no unconditional basic sequences. In our spaces $\mathcal{X}_\al$ every bounded operator $T$ is split into its diagonal part $D_T$ and its strictly singular part $S_T$.

On strictly singular operators between separable Banach spaces

Let $X$ and $Y$ be separable Banach spaces and denote by $\sss\sss(X,Y)$ the subset of $\llll(X,Y)$ consisting of all strictly singular operators. We study various ordinal ranks on the set $\sss\sss(X,Y)$. Our main results are summarized as follows. Firstly, we define a new rank $\rs$ on $\sss\sss(X,Y)$. We show that $\rs$ is a co-analytic rank and that dominates the rank $\varrho$ introduced by Androulakis, Dodos, Sirotkin and Troitsky [Israel J. Math., 169 (2009), 221-250]. Secondly, for every $1\leq p<+\infty$ we construct a Banach space $Y_p$ with an unconditional basis such that $\sss\sss…

Banach空间上非范数达算子集合中的线性结构

Linear structures in the set of non-norm-attaining operators on Banach spaces

研究Banach空间上非范数达算子集合中的线性结构，证明非空集合在空间可性、最大空间可性及(α,β)-空间可性下的结果，扩展了已知结论。

Delta点及其对Banach空间几何的意义

Delta-points and their implications for the geometry of Banach spaces

该研究在Banach空间几何领域取得多项进展：构造具Δ-点的超自反空间，证明任意无限维Banach空间可重赋范出Δ-点，建立Δ-点与对偶空间的关联并给出若干不含Δ-点的空间类，还给出Lipschitz-free空间中Δ-点的度量刻画并解决相关公开问题。

自反 Banach 空间的刻画

Characterizations of reflexive Banach spaces

本文综述自反 Banach 空间基于收敛性、弱紧性、仿射集和双正交系统的刻画，并讨论含 Schauder 基空间中与 $\ell^1$、$c_0$ 同构及非自反性的关系。

关于Bhatia-Šemrl性质、强次可微性和Banach空间上算子的本质范数

Connections between Essential Norm, Bhatia-Šemrl Property and Strong Subdifferentiability of Operators on Banach Spaces

研究Banach空间中算子的本质范数与Bhatia-Šemrl性质、强次可微性之间的关系，证明本质范数严格小于算子范数的条件，并提供相关算子示例。

Closed ideals of operators on the Tsirelson and Schreier spaces

Let $\mathscr{B}(X)$ denote the Banach algebra of bounded operators on $X$, where~$X$ is either Tsirelson's Banach space or the Schreier space of order $n$ for some $n\in\mathbb N$. We show that the lattice of closed ideals of~$\mathscr{B}(X)$ has a very rich structure; in particular $\mathscr{B}(X)$ contains at least continuum many maximal ideals. Our approach is to study the closed ideals generated by the basis projections. Indeed, the unit vector basis is an unconditional basis for each of the above spaces, so there is a basis projection $P_N\in\mathscr{B}(X)$ corresponding to each non-empt…