An arbitrarily distortable Banach space

Note on distortion and Bourgain $\ell_1$ index

The relation between different notions measuring proximity to $\ell_1$ and distortability of a Banach space is studied. The main result states that a Banach space, whose all subspaces have Bourgain $\ell_1$ index greater than $ω^α$, $α<ω_1$, contains either an arbitrary distortable subspace or an $\ell_1^α$-asymptotic subspace.

A $c\_0$-saturated Banach space with no long unconditional basic sequences

We present a Banach space $\mathfrak X$ with a Schauder basis of length $ω\_1$ which is saturated by copies of $c\_0$ and such that for every closed decomposition of a closed subspace $X=X\_0\oplus X\_1$, either $X\_0$ or $X\_1$ has to be separable. This can be considered as the non-separable counterpart of the notion of hereditarily indecomposable space. Indeed, the subspaces of $\mathfrak X$ have ``few operators'' in the sense that every bounded operator $T:X \to \mathfrak{X}$ from a subspace $X$ of $\mathfrak{X}$ into $\mathfrak{X}$ is the sum of a multiple of the inclusion and a $ω\_1$-sin…

Distorting Mixed Tsirelson Spaces

Any regular mixed Tsirelson space $T(θ_n,S_n)_{\N}$ for which $\frac{θ_n}{θ^n} \to 0$, where $θ=\lim_n θ_n^{1/n}$, is shown to be arbitrarily distortable. Certain asymptotic $\ell_1$ constants for those and other mixed Tsirelson spaces are calculated. Also a combinatorial result on the Schreier families $(S_α)_{α< ω_1}$ is proved and an application is given to show that for every Banach space $X$ with a basis $(e_i)$, the two $Δ$-spectrums $Δ(X)$ and $Δ(X,(e_i))$ coincide.

\ell^1-spreading models in subspaces of mixed Tsirelson spaces

We investigate the existence of higher order \ell^1-spreading models in subspaces of mixed Tsirelson spaces. For instance, we show that the following conditions are equivalent for the mixed Tsirelson space X=T[(θ_n,S_n)_{n=1}^{\infty}] (1)Every block subspace of $X$ contains an \ell^1-S_ω-spreading model, (2)The Bourgain \ell^1-index I_b(Y) = I(Y) > ω^ω for any block subspace Y of X, (3)\lim_m\limsup_nθ_{m+n}/θ_n > 0 and every block subspace Y of X contains a block sequence equivalent to a subsequence of the unit vector basis of X. Moreover, if one (and hence all) of these conditions holds, th…

Stability Results Of Small Diameter Properties In Banach Spaces

The geometric notion of huskability initiated and developed in [B3], [BR] ,[EW], [GM] was subsequently extensively studied in the context of dentability and Radon Nikodym Property in [GGMS]. In this work, we introduce a new geometric property of Banach space, the Ball Huskable Property ($BHP$), namely, the unit ball has relatively weakly open subsets of arbitrarily small diameter. We compare this property to two related geometric properties, $BSCSP$ namely, the unit ball has convex combination of slices of arbitrarily small diameter and $BDP$ namely, the closed unit ball has slices of arbitrar…

Banach空间的粗嵌入比

Coarse Embeddability Ratios of Banach Spaces

本文引入粗嵌入比作为Banach空间粗嵌入和一致嵌入的数值判据，利用非线性理论计算若干重要空间类的粗嵌入比，并构造粗嵌入比任意大的空间对，否定回答了Rosendal的一个公开问题。

Small Combination of Slices, Dentability and Stability Results Of Small Diameter Properties In Banach Spaces

In this work we study three different versions of small diameter properties of the unit ball in a Banach space and its dual. The related concepts for all closed bounded convex sets of a Banach space was initiated and developed in \cite{B3}, \cite{BR} ,\cite{EW}, \cite{GM} was extensively studied in the context of dentability, huskability, Radon Nikodym Property and Krein Milman Property in \cite{GGMS}. We introduce the the Ball Huskable Property ($BHP$), namely, the unit ball has relatively weakly open subsets of arbitrarily small diameter. We compare this property to two related properties, $…

Extreme differences between weakly open subsets and convex combinations of slices in Banach spaces

We show that every Banach space containing isomorphic copies of $c_0$ can be equivalently renormed so that every nonempty relatively weakly open subset of its unit ball has diameter 2 and, however, its unit ball still contains convex combinations of slices with diameter arbitrarily small, which improves in a optimal way the known results about the size of this kind of subsets in Banach spaces.

Modified mixed Tsirelson spaces

We study the modified and boundedly modified mixed Tsirelson spaces $T_M[({\cal F}_{k_n},θ_n)_{n=1}^{\infty }]$ and $T_{M(s)}[({\cal F}_{k_n},θ_n)_{n=1}^{\infty }]$ respectively, defined by a subsequence $({\cal F}_{k_n})$ of the sequence of Schreier families $({\cal F}_n)$. These are reflexive asymptotic $\ell_1$ spaces with an unconditio- nal basis $(e_i)_i$ having the property that every sequence $\{ x_i\}_{i=1}^n$ of normalized disjointly supported vectors contained in $\langle e_i\rangle_{i=n}^{\infty }$ is equivalent to the basis of $\ell_1^n$. We show that if $\limθ_n^{1/n}=1$ then the …

Delta- and Daugavet-points in Banach spaces

A $Δ$-point $x$ of a Banach space is a norm one element that is arbitrarily close to convex combinations of elements in the unit ball that are almost at distance $2$ from $x$. If, in addition, every point in the unit ball is arbitrarily close to such convex combinations, $x$ is a Daugavet-point. A Banach space $X$ has the Daugavet property if and only if every norm one element is a Daugavet-point. We show that $Δ$- and Daugavet-points are the same in $L_1$-spaces, $L_1$-preduals, as well as in a big class of Müntz spaces. We also provide an example of a Banach space where all points on the uni…

Big slices versus big relatively weakly open subsets in Banach spaces

We study the unknown differences between the size of slices and relatively weakly open subsets of the unit ball in Banach spaces. We show that every Banach space containing isomorphic copies of $c_0$ can be equivalently renormed so that every slice of its unit ball has diameter 2 and satisfying that its unit ball contains nonempty relatively weakly open subsets with diameter arbitrarily small, which answers an open problem and stresses the differences of diameter between slices and relatively weakly open subsets of the unit ball in Banach spaces.

Normal property, Jamenson property, CHIP and linear regularity for an infinite system of convex sets in Banach spaces

In this paper, we study different kinds of normal properties for infinite system of arbitrarily many convex sets in a Banach space and provide the dual characterization for the normal property in terms of the extended Jamenson property for arbitrarily many weak*-closed convex cones in the dual space. Then, we use the normal property and the extended Jamenson property to study CHIP, strong CHIP and linear regularity for the infinite case of arbitrarily many convex sets and establish equivalent relationship among these properties. In particular, we extend main results in [3] on normal property, …

具有Daugavet性质的严格凸且光滑的Banach空间

A strictly convex and smooth Banach space with the Daugavet property

本文在$L_1[0,1]$上构造了接近通常范数的等价范数，使空间严格凸且光滑，且对偶范数具有Daugavet性质，解决了长期未解问题。

Spectral-free methods in the theory of hereditarily indecomposable Banach spaces

We give new and simple proofs of some classical properties of hereditarily indecomposable Banach spaces, including the result by W. T. Gowers and B. Maurey that a hereditarily indecomposable Banach space cannot be isomorphic to a proper subspace of itself. These proofs do not make use of spectral theory and therefore, they work in real spaces as well as in complex spaces. We use our method to prove some new results. For example, we give a quantitative version of the latter result by Gowers and Maurey and deduce that Banach spaces that are isometric to all of their subspaces should have an unco…

Bidual octahedral renormings and strong regularity in Banach spaces

We prove that every separable Banach space containing $\ell_1$ can be equivalently renormed so that its bidual space is octahedral, which answers, in the separable case, a question by Godefroy in 1989. As a direct consequence, we obtain that every dual Banach space, with a separable predual, failing to be strongly regular (that is, without convex combinations of slices with diameter arbitrarily small for some closed, convex and bounded subset) can be equivalently renormed with a dual norm to satisfy the strong diameter two property (that is, such that every convex combination of slices in its …

On weakly almost square Banach spaces

We prove some results on weakly almost square Banach spaces and their relatives. On the one hand, we discuss weak almost squareness in the setting of Banach function spaces. More precisely, let $(Ω,Σ)$ be a measurable space, let $E$ be a Banach lattice and let $ν:Σ\to E^+$ be a non-atomic countably additive measure having relatively norm compact range. Then the space $L_1(ν)$ is weakly almost square. This result applies to some abstract Cesàro function spaces. Similar arguments show that the Lebesgue-Bochner space $L_1(μ,Y)$ is weakly almost square for any Banach space~$Y$ and for any non-atom…

Small Diameter Properties In Ideals of Banach Spaces

A Banach space has the ball huskable property ($BHP$) if the closed unit ball has weakly open sets of arbitrarily small diameter. We can analogously define $w^*$-$BHP$ in the dual space. In this short note, we study these properties in the context of ideals in Banach spaces. The notion of an ideal, was introduced by Godefroy, Kalton and Saphar. We show that if a Banach space $X$ has $BHP,$ then any $M$-ideal of $X$ also has $BHP.$ We further show that if $Y$ is an $M$-ideal of $X,$ then $Y^*$ has $w^*$-$BHP$ implies $X^*$ has $w^{*}$-$BHP.$ We use this result to prove that for a compact Hausdo…

Invariant half-spaces for rank-one perturbations

If T is a bounded linear operator acting on an infinite-dimensional Banach space, then there exists and operator F of rank at most one and arbitrarily small norm such that T-F has an invariant subspace of infinite dimension and codimension. This improves results of Tcaciuc and other authors.

平面巴拿赫-马祖尔紧致集的等边维数

Equilateral dimension of the planar Banach--Mazur compactum

本文证明了在巴拿赫-马祖尔距离下，平面对称凸体中存在任意大的等边集，并展示了构造的最优性。

齐次Banach空间作为$M(G)$上的Banach卷积模

Homogeneous Banach spaces as Banach convolution modules over $M(G)$

本文通过构造精细有界均匀单位分割，避免结构理论和Haar测度，建立了有界测度空间$M(G)$在局部紧Abel群$G$的齐次Banach空间上的自然卷积模结构，并证明了这些空间是本质$L^1(G)$-模。

Banach空间上线性余切Oseledets子空间的Hölder连续性

Hölder continuity of Oseledets subspaces for linear cocycles on Banach spaces

本文研究了Banach空间上线性余切的Oseledets子空间，在可逆与不可逆情形下证明了其Hölder连续性，推广了已有结果。

Banach 空间中的渐近光滑性、集中性质及其应用

Asymptotic smoothness, concentration properties in Banach spaces and applications

本文证明了 Hamming 图上 Lipschitz 映射的集中性质在 $\ell_p$-和下具有最优稳定性，并由此构造出 Szlenk 指标任意高且具有集中性质但无渐近光滑性的 Banach 空间实例。

The algebra of bounded type holomorphic functions on the ball

We study the spectrum $M_b(U)$ of the algebra of bounded type holomorphic functions on a complete Reinhardt domain in a symmetrically regular Banach space $E$ as an analytic manifold over the bidual of the space. In the case that $U$ is the unit ball of $\ell_p$, $1<p<\infty$, we prove that each connected component of $M_b(B_{\ell_p})$ naturally identifies with a ball of a certain radius. We also provide estimates for this radius and in many natural cases we have the precise value. As a consequence, we obtain that for connected components different from that of evaluations, these radii are str…

$\left(φ_1, φ_2\right)-$Variational principle

In this paper we prove that if $X $ is a Banach space, then for every lower semi-continuous bounded below function $f, $ there exists a $\left(φ_1, φ_2\right)-$convex function $g, $ with arbitrarily small norm, such that $f + g $ attains its strong minimum on $X. $ This result extends some of the well-known varitional principles as that of Ekeland [18], that of Borwein-Preiss [6] and that of Deville-Godefroy-Zizler [14, 15].

非一致连续的最近点映射

Non-uniformly continuous nearest point maps

构造一个局部一致凸的Banach空间，其上到任意紧凸子集的最近点映射连续但非一致连续，并证明任意无限维可分Banach空间在Banach-Mazur距离下可任意接近此类空间。

非凸变分问题的扰动方法

Perturbation method for non-convex variational problems

针对具有二次动能项和下半连续势的Banach空间变分问题，通过将空间范数替换为与原范数任意接近的等价范数，恢复极小元的存在性，并将Ivanov和Zlateva的结果从凸情形推广到下半连续情形。

单位球中具有小的弱开子集且拥有大量 Daugavet 点和 Δ 点集合的 Banach 空间

Banach spaces with small weakly open subsets of the unit ball and massive sets of Daugavet and $Δ$-points

本文在 $L_\infty[0,1]$ 上构造等价范数，使单位球兼具任意小直径弱开集、弱稠密 Daugavet 点与赋范 ccw Δ 点，同时不满足直径局部直径 2 性质。

The isomorphic Kottman constant of a Banach space

We show that the Kottman constant $K(\cdot)$, together with its symmetric and finite variations, is continuous with respect to the Kadets metric, and they are log-convex, hence continuous, with respect to the interpolation parameter in a complex interpolation schema. Moreover, we show that $K(X)\cdot K(X^*)\geqslant 2$ for every infinite-dimensional Banach space $X$. We also consider the isomorphic Kottman constant (defined as the infimum of the Kottman constants taken over all renormings of the space) and solve the main problem left open in [CaGoPa17], namely that the isomorphic Kottman const…

Banach Spaces from Barriers in High Dimensional Ellentuck Spaces

A new hierarchy of Banach spaces $T_k(d,θ)$, $k$ any positive integer, is constructed using barriers in high dimensional Ellentuck spaces \cite{DobrinenJSL15} following the classical framework under which a Tsirelson type norm is defined from a barrier in the Ellentuck space \cite{Argyros/TodorcevicBK}. The following structural properties of these spaces are proved. Each of these spaces contains arbitrarily large copies of $\ell_\infty^n$, with the bound constant for all $n$. For each fixed pair $d$ and $θ$, the spaces $T_k(d,θ)$, $k\ge 1$, are $\ell_p$-saturated, forming natural extensions of…

Compact Hölder retractions and nearest point maps

In this paper, two main results concerning uniformly continuous retractions are proved. First, an $α$-Hölder retraction from any separable Banach space onto a compact convex subset whose closed linear span is the whole space is constructed for every positive $α<1$. This constitutes a positive solution to a Hölder version of a question raised by Godefroy and Ozawa. In fact, compact convex sets are found to be absolute $α$-Hölder retracts under certain assumption of flatness. Second, we provide an example of a strictly convex Banach space $X$ arbitrarily close to $\ell_2$ (for the Banach Mazur d…

Banach空间中线性算子的虚能级与虚态及其在薛定谔算子中的应用

Virtual levels and virtual states of linear operators in Banach spaces. Applications to Schroedinger operators

本文提出Banach空间中虚能级的统一刻画方法，证明其等价于存在虚态、缺乏极限吸收原理及小扰动产生特征值，并应用于任意维非自伴非局部势薛定谔算子的阈值预解估计。

There is no bound on sizes of indecomposable Banach spaces

Assuming the generalized continuum hypothesis we construct arbitrarily big indecomposable Banach spaces. i.e., such that whenever they are decomposed as $X\oplus Y$, then one of the closed subspaces $X$ or $Y$ must be finite dimensional. It requires alternative techniques compared to those which were initiated by Gowers and Maurey or Argyros with the coauthors. This is because hereditarily indecomposable Banach spaces always embed into $\ell_\infty$ and so their density and cardinality is bounded by the continuum and because dual Banach spaces of densities bigger than continuum are decomposabl…

半射性巴拿赫格局

Semiprojective Banach lattices

研究半射性巴拿赫格局的性质，证明Banach格C(X)半射性当且仅当X是绝对邻域拉回，且展示无限可数的ℓ₁和的1+项目性巴拿赫格局为半射性但不一定是1+项目性，同时证明ℓ_p和L_p空间不满足半射性。

Sharp quantitative nonembeddability of the Heisenberg group into superreflexive Banach spaces

Let $\H$ denote the discrete Heisenberg group, equipped with a word metric $d_W$ associated to some finite symmetric generating set. We show that if $(X,\|\cdot\|)$ is a $p$-convex Banach space then for any Lipschitz function $f:\H\to X$ there exist $x,y\in \H$ with $d_W(x,y)$ arbitrarily large and \begin{equation}\label{eq:comp abs} \frac{\|f(x)-f(y)\|}{d_W(x,y)}\lesssim \left(\frac{\log\log d_W(x,y)}{\log d_W(x,y)}\right)^{1/p}. \end{equation} We also show that any embedding into $X$ of a ball of radius $R\ge 4$ in $\H$ incurs bi-Lipschitz distortion that grows at least as a constant multipl…

某些ℓ_p求和与锥体的精确Banach-Mazur距离

Exact Banach-Mazur distances of certain $\ell_p$-sums and cones

本文研究了有限维实normed空间的ℓ_p直接和与相关凸体锥构造的Banach-Mazur距离，通过最近的最优Banach-Mazur位置特性，推导出从X_1⊕_p…⊕_pX_k到欧几里得空间的距离公式，并探讨了单锥与双锥的几何性质及距离关系。