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It is shown that a script L_infty-space with separable dual constructed by Bourgain and Delbaen has small Szlenk index and thus does not have a quotient isomorphic to C(omega^omega). It follows that this is a script L_infty-space which is the same size as c_0 in the sense of the Szlenk index but does not contain c_0. This has some consequences in the theory of uniform homeomorphism of Banach spaces.
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We prove that, for a suitable choice of real numbers $a, b$, every operator from $\ell_2$ to $X_{a,b}$ and from $X_{a,b}$ to $\ell_2$ must be compact, where $X_{a,b}$ is the Bourgain- Delbaen's space.
We begin by giving a detailed exposition of the original Bourgain-Delbaen construction and the generalised construction due to Argyros and Haydon. We show how these two constructions are related, and as a corollary, are able to prove that there exists some $δ> 0$ and an uncountable set of isometries on the original Bourgain-Delbaen spaces which are pairwise distance $δ$ apart. We subsequently extend these ideas to obtain our main results. We construct new Banach spaces of Bourgain-Delbaen type, all of which have $\ell_1$ dual. The first class of spaces are HI and possess few, but not very few …
Motivated by a problem stated by S.A.Argyros and Th. Raikoftsalis, we introduce a new class of Banach spaces. Namely, for a sequence of separable Banach spaces $(X_n,\|\cdot\|_n)_{n\in\mathbb{N}}$, we define the Bourgain Delbaen $\mathcal{L}^{\infty}$-sum of the sequence $(X_n,\|\cdot\|_n)_{n\in\mathbb{N}}$ which is a Banach space $\mathcal{Z}$ constructed with the Bourgain-Delbaen method. In particular, for every $1\leq p<\infty$, taking $X_n=\ell_p$ for every $n\in\mathbb{N}$ the aforementioned space $\mathcal{Z}_p$ is strictly quasi prime and admits $\ell_p$ as a complemented subspace. We s…
The space $\mathscr B_{mt}[(m_j)_j,(n_j)_j]$ is a Bourgain-Delbaen space modelled on a mixed Tsirelson space $T[(m_j)_j,(n_j)_j]$ and is a slight modification of $\mathfrak B_{\text{mt}}[(m_j)_j,(n_j)_j]$, a space defined by S. Argyros and R. Haydon. We prove that in every infinite dimensional subspace of $\mathscr B_{mt}[(m_j)_j,(n_j)_j]$ there exists a basic sequence equivalent to a sequence of weighted basis averages of increasing length from $T[(m_j)_j,(n_j)_j]$. We remark that the same is true for the original space $\mathfrak B_{\text{mt}}[(m_j)_j,(n_j)_j]$.
Based on a construction method introduced by J. Bourgain and F. Delbaen, we give a general definition of a Bourgain-Delbaen space and prove that every infinite dimensional separable $\mathcal{L}_\infty$-space is isomorphic to such a space. Furthermore, we provide an example of a $\mathcal{L}_\infty$ and asymptotic $c_0$ space not containing $c_0$.
Operator Algebras of Bourgain Delbaen Spaces: Realization, Rigidity, and Ideal Structure
本文研究了Calkin代数及其作为紧算子商的结构，通过保持几何和拓扑信息实现交换C*代数。证明了对于每个紧度量空间K，存在反射Banach空间X_C(K)，其Calkin代数同构于C(K)。同时建立了稳定性、局部化原理、Hölder连续性、刚性定理及理想结构分类等贡献。
We show that the class of all Banach spaces which are isomorphic to $ c_{0} $ is a complete analytic set with respect to the Effros Borel structure of separable Banach spaces. The proof employs a recent Bourgain-Delbaen construction by Argyros, Gasparis and Motakis.
We construct a Bourgain-Delbaen $\mathscr{L}_\infty$-space $\mathfrak{X}_{Kus}$ with strongly heterogenous structure: any bounded operator on $\mathfrak{X}_{Kus}$ is a compact perturbation of a multiple of the identity, whereas the space $\mathfrak{X}_{Kus}$ is saturated with unconditional basic sequences.
Mazur's Separable Quotient Problem for Nonseparable Bourgain-Pisier $\mathscr{L}_\infty$-Spaces
本文解决马祖尔问题，证明满足特定条件的L_∞空间存在c_0商，通过构造性方法和反例验证假设的必要性。
A hereditarily indecomposable Banach space $\mathfrak{X}_{\mathfrak{nr}}$ is constructed that is the first known example of a $\mathscr{L}_\infty$-space not containing $c_0$, $\ell_1$, or reflexive subspaces and answers a question posed by J. Bourgain. Moreover, the space $\mathfrak{X}_{\mathfrak{nr}}$ satisfies the "scalar-plus-compact" property and it is the first known space without reflexive subspaces having this property. It is constructed using the Bourgain-Delbaen method in combination with a recent version of saturation under constraints in a mixed-Tsirelson setting. As a result, the s…
Bourgain-Morrey sequence spaces: structural properties, relations to classical $\ell^{p}$ spaces and duality
本文研究了离散Bourgain-Morrey序列空间的结构性质、与经典ℓ^p空间的关系及对偶性，揭示了其可分性、嵌入关系及对偶空间的构造。
On the mixed Bourgain-Morrey spaces
本文引入混合Bourgain-Morrey空间，证明了多种算子在其上的有界性，建立相关Littlewood-Paley理论，给出小波刻画等应用，描述对偶空间并得出其自反性。
Given a Banach space $E$ with a supremum-type norm induced by a collection of operators, we prove that $E$ is a dual space and provide an atomic decomposition of its predual. We apply this result, and some results obtained previously by one of the authors, to the function space $\mathcal{B}$ introduced recently by Bourgain, Brezis, and Mironescu. This yields an atomic decomposition of the predual $\mathcal{B}_\ast$, the biduality result that $\mathcal{B}_0^\ast = \mathcal{B}_\ast$ and $\mathcal{B}_\ast^\ast = \mathcal{B}$, and a formula for the distance from an element $f \in \mathcal{B}$ to $…
Mixed Bourgain-Morrey spaces and their applications to boundedness of operators
本文引入混合Bourgain-Morrey空间并获得其预对偶空间，证明了Hardy-Littlewood极大算子、迭代极大算子、分数积分算子和奇异积分算子在这些空间上的有界性，并讨论了这些空间的对偶性和 reflexivity。
We construct a class of minimal trees and use these trees to establish a number of coloring theorems on general trees. Among the applications of these trees and coloring theorems are quantification of the Bourgain $\ell_p$ and $c_0$ indices, dualization of the Bourgain $c_0$ index, establishing sharp positive and negative results for constant reduction, and estimating the Bourgain $\ell_p$ index of an arbitrary Banach space $X$ in terms of a subspace $Y$ and the quotient $X/Y$.
The preduals of Banach space valued Bourgain-Morrey spaces
本文引入Banach空间值Bourgain-Morrey空间，证明其预对偶为块空间，并研究其完备性、对偶、反射性及Hardy-Littlewood极大算子的有界性。
In this paper, we extend the definition of fractional gradients found in Mazowiecka-Schikorra to tempered distributions on $\R^n$, introduce associated regularisation procedures and establish some first regularity results for distributional fractional gradients in $L^{1}_{od}$. The key feature is the introduction of a suitable space of off-diagonal Schwarz functions $\mathcal{S}_{od}(\R^{2n})$, allowing for a dual definition of the fractional gradient on an appropriate space of distributions $\mathcal{S}^{\prime}_{od}(\R^{2n})$ by means of fractional divergences defined on $\mathcal{S}_{od}(\R…
Bourgain-Morrey-Lorentz spaces and operators on them
引入Bourgain-Morrey-Lorentz空间，通过块空间刻画其前对偶，利用对偶性证明多种算子有界性，并给出BMO函数及交换子紧性的刻画。
We present some extensions of classical results that involve elements of the dual of Banach spaces, such as Bishop-Phelp's theorem and James' compactness theorem, but restricting to sets of functionals determined by geometrical properties. The main result, which answers a question posed by F. Delbaen, is the following: Let $E$ be a Banach space such that $(B_{E^\ast}, ω^\ast)$ is convex block compact. Let $A$ and $B$ be bounded, closed and convex sets with distance $d(A,B) > 0$. If every $x^\ast \in E^\ast$ with \[ \sup(x^\ast,B) < \inf(x^\ast,A) \] attains its infimum on $A$ and its supremum …
On Subspaces of Indecomposable Banach Spaces
本文刻画了可嵌入不可分解Banach空间的Banach空间类，证明其包含所有密度不超过连续统且不以ℓ∞为商空间的Banach空间，并改进构造方法以涵盖部分以ℓ∞为商空间的情形，为Argyros–Haydon提出的问题提供新见解。
Let $K$ be an $n$-dimensional symmetric convex body with $n \ge 4$ and let $K\dual$ be its polar body. We present an elementary proof of the fact that $$(\Vol K)(\Vol K\dual)\ge \frac{b_n^2}{(\log_2 n)^n},$$ where $b_n$ is the volume of the Euclidean ball of radius 1. The inequality is asymptotically weaker than the estimate of Bourgain and Milman, which replaces the $\log_2 n$ by a constant. However, there is no known elementary proof of the Bourgain-Milman theorem.
Bourgain--Brezis selection maps for Hodge systems in Besov and Triebel--Lizorkin spaces
该论文通过结合无迹 Beurling--Ahlfors 变换与非线性对偶方法，在 Besov 和 Triebel--Lizorkin 空间中扩展了 Hodge 系统的 Bourgain--Brezis 选择映射的适用范围，显著拓宽了已知解范围。
Let $X$ be a Banach space and $Γ\subseteq X^*$ a total linear subspace. We study the concept of $Γ$-integrability for $X$-valued functions $f$ defined on a complete probability space, i.e. an analogue of Pettis integrability by dealing only with the compositions $\langle x^*,f \rangle$ for $x^*\in Γ$. We show that $Γ$-integrability and Pettis integrability are equivalent whenever $X$ has Plichko's property ($\mathcal{D}'$) (meaning that every $w^*$-sequentially closed subspace of $X^*$ is $w^*$-closed). This property is enjoyed by many Banach spaces including all spaces with $w^*$-angelic dual…
We show that the space of all bounded derivations from the disc algebra into its dual can be identified with the Hardy space $H^1$; using this, we infer that all such derivations are compact. Also, given a fixed derivation $D$, we construct a finite, positive Borel measure $μ_D$ on the closed disc, such that $D$ factors through $L^2(μ_D)$. Such a measure is known to exist, for any bounded linear map from the disc algebra to its dual, by results of Bourgain and Pietsch, but these results are highly non-constructive.
From Sharp Self-Improvement to Exact Perimeter Formulas in Bourgain--Brezis--Mironescu Spaces
本文研究Bourgain--Brezis--Mironescu空间的$L^q$变体，刻画了函数与特征函数的端点行为，并建立了精确周长公式的收敛性及其维度限制。
Let $1\leq p,q < \infty$ and $1\leq r \leq \infty$. We show that the direct sum of mixed norm Hardy spaces $\big(\sum_n H^p_n(H^q_n)\big)_r$ and the sum of their dual spaces $\big(\sum_n H^p_n(H^q_n)^*\big)_r$ are both primary. We do so by using Bourgain's localization method and solving the finite dimensional factorization problem. In particular, we obtain that the spaces $\big(\sum_{n\in \mathbb N} H_n^1(H_n^s)\big)_r$, $\big(\sum_{n\in \mathbb N} H_n^s(H_n^1)\big)_r$, as well as $\big(\sum_{n\in \mathbb N} BMO_n(H_n^s)\big)_r$ and $\big(\sum_{n\in \mathbb N} H^s_n(BMO_n)\big)_r$, $1 < s < \…
We investigate the isomorphic structure of the Cesàro spaces and their duals, the Tandori spaces. The main result states that the Cesàro function space $Ces_{\infty}$ and its sequence counterpart $ces_{\infty}$ are isomorphic, which answers to the question posted in \cite{AM09}. This is rather surprising since $Ces_{\infty}$ has no natural lattice predual similarly as the known Talagrand's example \cite{Ta81}. We prove that neither $ces_{\infty}$ is isomorphic to $l_{\infty}$ nor $Ces_{\infty}$ is isomorphic to the Tandori space $\widetilde{L_1}$ with the norm $\|f\|_{\widetilde{L_1}}= \|\wide…
We construct uniformly bounded solutions for the equations $\text{div}\, U=f$ and $\text{curl}\, U=F$ in the critical cases $f \in L^d(T^d,R)$, and respectively, $F \in L^3(T^3,R^3)$. Criticality in this context, manifests itself by the lack of linear solution operator mapping $L^d$ to $L^\infty(T^d)$, Bourgain & Brezis \cite{BB03,BB07}. Thus, the intriguing aspect here is that although the problems are linear, the construction of their solution is not. Our constructions are special cases of a general framework for solving linear equations of the form $T\, U=f$, where $T$ is a linear operator …
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