2021
Jacopo Somaglia (University of Milan, Italy)
Norming M-bases in non-separable Banach spaces
04.11.2021
Abstract
In the first part of the talk we provide a brief overview on M-bases in non-separable Banach spaces, focusing on norming M-bases and some related classes of Banach spaces. Then, some recent results in this context will be discussed. In particular, a negative answer to the following longstanding open question will be presented: Must every Asplund Banach space with a norming M-basis be weakly compactly generated? Part of the contents of the talk are included in a recent joint work with P. Hájek, T. Russo, and S. Todorčević.
Javier Falcó Benavent (Universitat de València, Spain)
Norm attaining group invariant functionals
29.10.2021
Abstract
We will review some basic results of the norm attaining theory. We will also present some recent results in this theory in the case that the functionals are invariant under a group action on the space. The most basic examples of group invariant functionals are functionals on R2 that are invariant under permutations, that is, f(x,y)=f(y,x) for all x,y∈R. We will conclude this talk by presenting some group invariant results in the setting of norm attaining operators.
Ramón J. Aliaga (Universidad Politecnica de Valencia, Spain)
Extreme points of the unit ball in Lipschitz-free spaces
22.10.2021
Abstract
Lipschitz-free Banach spaces are currently the object of intensive scrutiny, yet many of its basic geometric properties remain shrouded in mystery. In this talk, we will discuss the problem of determining the extreme points of the unit ball of the Lipschitz-free space F(M) over a given metric space M. The problem, which is still open in general, has been addressed by authors such as García-Lirola, Pernecká, Petitjean, Procházka, Rueda Zoca, and Weaver. We will review their contributions and go into some detail over the recent solution for the case where M is compact (see https://arxiv.org/abs/2102.01219), with focus on the main techniques for handling the general problem.
Geunsu Choi (Dongguk University, Seoul, South Korea)
The Birkhoff-James orthogonality and norm attainment for multilinear maps
15.10.2021
Abstract
Very recently, motivated by the result of Bhatia and Šemrl which characterizes the Birkhoff-James orthogonality of operators on a finite dimensional Hilbert space in terms of norm attaining points, the Bhatia-Šemrl property was introduced. In this talk, we investigate the denseness of the set of multilinear maps with the Bhatia-Šemrl property which is contained in the set of norm attaining ones. Contrary to the most of previous results which were shown for operators on real Banach spaces, we prove the denseness for multilinear maps on some complex Banach spaces. We also show that the denseness of operators does not hold when the domain space is c_0 for arbitrary range. Moreover, we find plenty of Banach spaces Y such that only the zero operator has the Bhatia-Šemrl property in the space of operators from c_0 to Y.
Alberto Salguero Alarcón (Universidad de Extremadura, Badajoz, Spain)
Classification of twisted sums of c_0(I)
08.10.2021
Abstract
A twisted sum of two Banach spaces X and Y is another space Z containing Y as a closed subspace so that Z/Y = X. In this talk we focus on twisted sums of spaces of continuos functions on compact spaces. We first display some classical examples, and then we move to study the also standard spaces C(K_A), where K_A is the compact space generated by an almost-disjoint family A of subsets of N. They are twisted sums of c_0 and c_0(I), and this fact will prove essential in answering the following question: if C(K_A) is isomorphic to C(K_B), then... what
Stefano Ciaci (University of Tartu, Estonia)
Attaining strong diameter two property for infinite cardinals
01.10.2021
Abstract
A Banach space is said to have the strong diameter 2 property (SD2P) if every finite convex combination of slices of the unit ball has diameter 2 and it is said to have the attaining SD2P (ASD2P) if the diameter is attained by two elements of the unit sphere. What happens if we consider convex series of slices instead?
In order to answer this question, we extend the properties SD2P and ASD2P to infinite cardinals and we establish their dual connections with octahedral norms and Banach spaces failing the (−1)-ball-covering property. We also provide examples and, in particular, we characterize C(K) spaces and L1(μ) spaces satisfying the countable analogue of the ASD2P.
Triinu Veeorg (University of Tartu, Estonia)
Daugavet- and Delta-points in Lipschitz-free spaces
03.06.2021
Abstract
In this talk we provide two characterizations for Daugavet-points in Lipschitz-free spaces that work for any metric space $M$. Furthermore, we apply this result to construct an example of a metric space $M$ such that the corresponding Lipschitz-free space $\mathcal{F}(M)$ has the Radon--Nikod\'ym property and also contains a Daugavet-point. As a last part of this talk we provide a characterization for $\Delta$-points among convex combinations of molecules. Our main results generalize the ones from a recent preprint by Mingu Jung and Abraham Rueda Zoca (Daugavet points and Δ-points in Lipschitz-free spaces, to appear in Studia Math.).
Slides
Pedro L. Kaufmann (Federal University of São Paulo, Brazil)
Lipschitz-free spaces over compact Lipschitz manifolds
27.05.2021
Abstract
We will show that the Lipschitz-free space over a compact Lipschitz manifold of dimension d is isomorphic to the Lipschitz-free space over \mathbb{R}^d. Some tools used in the proof, as well as related problems and applications, will be discussed.
Andre Ostrak (University of Tartu, Estonia)
The weak-star symmetric strong diameter 2 property in Lipschitz spaces
20.05.2021
Abstract
It is known that a Banach space is octahedral if and only if its dual has the weak-star strong diameter 2 property (w*-SD2P). Furthermore, in [PR] it was shown that a Lipschitz-free space is octahedral if and only if the corresponding metric space has the long trapezoid property. In this talk, we focus on the main results of two recent papers [O1], [O2], that provided similar characterisations to the weak-star symmetric strong diameter 2 property (w*-SSD2P) in Lipschitz spaces. In particular, we show that the w*-SSD2P is strictly stronger that the w*-SD2P in Lipschitz spaces, and we give a characterisation of the w*-SD2P of the Lipshitz space Lip₀(M) in terms of the corresponding metric space M and the corresponding Lipschitz-free space F(M).
[O1] A. Ostrak. Characterisation of the weak-star symmetric strong diameter 2 property in Lipschitz spaces. J. Math. Anal. Appl. 483 (2020), no. 2, 123630.
[O2] A. Ostrak. On the duality of the symmetric strong diameter 2 property in Lipschitz spaces. RACSAM 115, 78 (2021).
[PR] A. Prochazka, A. Rueda Zoca. A characterisation of octahedrality in Lipschitz-free spaces. Annales de l’Institut Fourier, 68 (2018) no. 2, 569-588.
Óscar Roldán Blay (University of Valencia, Spain)
The Bishop-Phelps-Bollob'as property for numerical radius and compact operators
13.05.2021
Abstract
In this talk, we will present the Bishop-Phelps-Bollob'as property for numerical radius restricted to the case of compact operators (abbreviated BPBp-nu for compact operators). Roughly speaking, a Banach space X has this property if, whenever we have a compact operator T:X→X with numerical radius 1 that almost attains its numerical radius at some state (x,x∗), there is a nearby compact operator S, also with numerical radius 1, that attains its numerical radius at a nearby state (y,y∗). We will first provide the necessary background to define this property and an initial wide list of Banach spaces satisfying the property. We will also see a tool that allows us to carry the property from some spaces to others, and we will use it to get more examples of spaces satisfying the property. Finally, we will see a visualization of the proof that all C0(L) spaces have the property, whenever L is locally compact and Hausdorff. The contents of this talk are from a recent joint work with Domingo García, Manuel Maestre and Miguel Martín [GMMR2021].
[GMMR2021] D. García, M. Maestre, M. Martín, Ó. Roldán, On the compact operators case of the Bishop-Phelps-Bollob\'as property for numerical radius. To appear in Results in Mathematics.
Mingu Jung (Pohang University of Science and Technology, Republic of Korea)
When do all bounded linear operators attain their norm?
06.05.2021
Abstract
In this talk, we provide necessary and sufficient conditions for the existence of non-norm-attaining operators in L(E,F). By using a theorem due to Pfitzner on James boundaries, we show that if there exists a relatively compact set K of L(E,F) (in the weak operator topology) such that 0 is an element of its closure (in the weak operator topology) but it is not in its norm closed convex hull, then we can guarantee the existence of an operator which does not attain its norm. This allows us to provide the following generalization of results due to Holub and Mujica. We also present a characterization of the Schur property in terms of norm-attaining operators.
Katriin Pirk (University of Tartu, Estonia)
Stability results of diametral diameter two properties
29.04.2021
Abstract
The diametral diameter two properties were introduced being inspired by two settings. On the one hand, there were the diameter two properties (see e.g. [ALN]) where specific subsets of the unit ball (slices, relatively weakly open subsets, convex combinations of slices) have diameter 2. On the other hand, there was the research on the spaces with bad projections (see [IK]), inspired by the well-known Daugavet property. The spaces with bad projections admit to the following characterisation: every slice S of the unit ball contains an almost diametral point for each norm-1 element in S. Obviously, that slice S has diameter 2.
Connecting these two ideas a system of diametral diameter two properties similar to the three-property-system of the diameter two properties was started in [BLR]. The afore-mentioned characterisation of the spaces with bad projections became the slice version and two other definitions were added (for resp. relatively weakly open subsets and convex combinations of relatively weakly open subsets). In [AHLP] an additional diametral diameter two property, called the convex diametral diameter two property, was introduced. These notions have been researched over the years by many mathematicians (see e.g., [AHLP], [AHNTT], [BLR], [HPP] etc.).
In this seminary talk we give an overview of the diametral diameter two properties with the focus on the recent stability results regarding these properties obtained in a joint work with Johann Langemets (see [LP]).
[AHLP] T. A. Abrahamsen, R. Haller, V. Lima, and K. Pirk, Delta- and Daugavet-points in Banach spaces, Proc. Edinb. Math. Soc. 63 (2020).
[AHNTT] T. A. Abrahamsen, P. Hájek, O. Nygaard, J. Talponen, and S. Troyanski, Diameter 2 properties and convexity, Studia Math. 232 (2016).
[ALN] T. A. Abrahamsen, V. Lima, and O. Nygaard, Remarks on diameter 2 properties, J. Convex Anal. 20 (2013).
[BLR] J. Becerra Guerrero, G. López-Pérez, and A. Rueda Zoca, Diametral Diameter Two Properties in Banach Spaces, J. Convex Anal. 25 (2018).
[HPP] R. Haller, K. Pirk, and M. Põldvere, Diametral strong diameter two property of Banach spaces is stable under direct sums with 1-norm, Acta Comment. Univ. Tartu Math. 20 (2016).
[IK] Y. Ivakhno and V. M. Kadets, Unconditional sums of spaces with bad projections, Visn. Khark. Univ., Ser. Mat. Prykl. Mat. Mekh. 645 (2004).
[LP] J. Langemets and K. Pirk, Stability results of diametral diameter two properties, Rev. R. Acad. Cienc. Exactas Fís. Nat. Ser. A Mat. RACSAM 115 (2021).
Miguel Martín (University of Granada, Spain)
Slicely countably determined sets, spaces, and operators
22.04.2021
Abstract
Slicely countably determined spaces (SCD spaces) were introduced by Aviles, Kadets, Martin, Meri, and Shepelska (Transactions of the American Mathematical Society 2010) as a topological property for separable Banach spaces which is satisfied by both Asplund spaces and spaces with the RNP (actually, by both spaces not containing ℓ1 and strongly regular spaces). The property is defined in terms of a property of the slices of convex bounded subsets and produce interesting classes of bounded linear operators (SCD and HSCD operators). These ideas have been successfully applied to get important consequences in the theory of numerical index one spaces, for the Daugavet property, and for the study of spear operators between Banach spaces.
The objective of this seminar is to present the main examples and applications of the theory, comment some recent developments and present some open problems.
Some related references are the following ones:
Aviles, Martin, Meri, Shepelska. Slicely countably determined Banach spaces. Trans. Amer. Math. Soc. 362 (2010), 4871–4900.
Kadets, Martin, Meri, Perez. Spear operators between Banach spaces. Lecture Notes in Math. 2205, 2018.
Kadets, Perez, Werner. Operations with slicely countably determined sets, Funct. Approx. 59 (2018), 77–98.
Luis C. García Lirola (University of Zaragoza, Spain)
The Weak Maximizing Property and asymptotic geometry of Banach spaces
15.04.2021
Abstract
Recently, a new property related to norm-attaining operators between Banach spaces has been introduced: the weak maximizing property (WMP). In this talk, we provide new sufficient conditions, based on the moduli of asymptotic uniform smoothness and convexity, which imply that a pair (X,Y) enjoys the WMP. This approach not only allows us to (re)obtain as a direct consequence that the pair (ℓp,ℓq) has the WMP, but also provides many more natural examples of pairs having a given maximizing property. This is part of a joint work with Colin Petitjean.
Nikita Leo (University of Tartu, Estonia)
Plasticity of the unit ball of c_0
08.04.2021
Abstract
In this talk we consider a challenging open problem of whether the unit ball of every Banach space is a plastic metric space (a metric space is called plastic if every non-expansive bijection from this space onto itself is an isometry). We are going to provide some insight into the problem and show what has been achieved so far. We also present a new result, which says that the unit ball of c0 has a property similar to plasticity - we show that a non-expansive bijection from the unit ball of c0 onto itself is an isometry, provided that the inverse map is continuous.
Hyung-Joon Tag (Dongguk University, Republic of Korea)
Diameter two properties in some vector-valued function spaces
01.04.2021
Abstract
In this talk, we introduce the space A(K,X) over a uniform algebra A on a compact Hausdorff space K. Every nonempty relatively weakly open subset of the unit ball of a vector-valued function space A(K,X) over an infinite dimensional uniform algebra has the diameter two. If X is uniformly convex, under the additional assumption of A⊗X⊂A(K,X), we show that Daugavet points and Δ-points on A(K,X) over a uniform algebra A are the same, and they are characterized by the norm-attainment at a limit point of the Shilov boundary of A. In addition, a sufficient condition for the convex diametral local diameter two property of A(K,X) is also provided.
https://arxiv.org/abs/2103.04012
Alicia Quero de la Rosa (University of Granada, Spain)
Numerical index with respect to an operator
25.03.2021
Abstract
The concept of numerical index was introduced by Lumer in 1968 in the context of the study and classification of operator algebras. This is a constant relating the norm and the numerical range of bounded linear operators on the space. More precisely, the numerical index of a Banach space X, n(X), is the greatest constant k≥0 such that k∥T∥≤sup{|x∗(Tx)|:x∗∈X∗,x∈X,∥x∗∥=∥x∥=x∗(x)=1} for every T∈L(X).
Recently, Ardalani introduced new concepts of numerical range, numerical radius, and numerical index, which generalize in a natural way the classical ones and allow to extend the setting to the context of operators between possibly different Banach spaces. Given a norm-one operator G∈L(X,Y) between two Banach spaces X and Y, the numerical index with respect to G, nG(X,Y), is the greatest constant k≥0 such that k∥T∥≤infδ>0sup{|y∗(Tx)|:y∗∈Y∗,x∈X,∥y∗∥=∥x∥=1,Rey∗(Gx)>1−δ} for every T∈L(X,Y).
In this talk, we will give an overview of the topic, analysing differences and similarities between these concepts, and presenting some classical and recent results in the area.
Tomasso Russo (Czech Academy of Sciences, Czech Technical University in Prague, Czech Republic)
(1+)-meters apart: Separated sets in Covid times
18.03.2021
Abstract
Riesz’ lemma, one of the most classical results in Functional Analysis and now proved in the first lectures of every course in the topic, asserts that the unit sphere SX of every infinite-dimensional Banach space X contains a sequence of points whose mutual distances are at least 1, thereby demonstrating the non-compactness of the unit ball in infinite dimensions. The, by now rather wide, topic of separated sets in Banach spaces can be safely considered to originate from such a lemma.
In our talk, we will survey some classical and recent results in the area, also pointing out some problems that remain open. We shall discuss the main ideas behind the proofs of selected results, as an illustration of some techniques in the area (both in the separable and the non-separable contexts). The talk is intended to be elementary and students-friendly, the unique real prerequisites being the definition of a Banach space and the Hahn–Banach theorem.
Rubén Medina (University of Granada, Spain; Czech Technical University in Prague, Czech Republic)
A characterization of the weak topology of the unit ball of purely atomic L_1 preduals
11.03.2021
Abstract
We study Banach spaces with a weak stable unit ball, that is, Banach spaces where every convex combination of relatively weakly open subsets of the unit ball is again relatively weakly open in its unit ball. A walkthrough of previous overall results will be done, as well as a more concise dedication to the newer results. The main result of the talk will be a characterization of the spaces with weak stable unit ball among the L_1-preduals.
Stefano Ciaci (University of Tartu, Estonia)
A characterization of Banach spaces containingℓ1(κ) via ball-covering properties
04.03.2021
Abstract
In 1989, G. Godefroy proved that a Banach space contains an isomorphic copy of ℓ1 if and only if it can be equivalently renormed to be octahedral. It is known that octahedral norms can be characterized by means of covering the unit sphere by a finite number of balls. This observation allows us to connect the theory of octahedral norms with ball-covering properties of Banach spaces introduced by L. Cheng in 2006. Following this idea, we extend G. Godefroy’s result to higher cardinalities. We prove that, for an infinite cardinal κ, a Banach space X contains an isomorphic copy of ℓ1(κ+) if and only if it can be equivalently renormed in such a way that its unit sphere cannot be covered by κ many open balls not containing αB_X, where α∈(0,1). We also investigate the relation between ball-coverings of the unit sphere and octahedral norms in the setting of higher cardinalities.
Antonio Pérez Hernández (UNED, Spain)
Spear operators between Banach spaces
25.02.2021
Abstract
The concept of spear operator is intimately related to the notions of numerical range, numerical radius and numerical index introduced for Banach spaces by Lumer and Bauer (1960s). Duncan et al. (1970s) showed that Banach spaces X having are characterized as those satisfying that for every T∈L(X) there is a modulus-one scalar θ such that
∥IdX+θT∥=1+∥T∥,
also providing the first examples and properties. The geometrical properties of these spaces have been extensively studied, searching for characterizations that do not involve operators, and leading to the introduction of new properties (lushness, alternative Daugavet property…) that have become useful in other settings.
More recently, Ardalani (2014) presented more general definitions of numerical range, numerical radius and numerical index in the context of operators between possibly different Banach spaces. In particular, he introduced the concept of and initiated the study of general spear operators: a norm-one operator G∈L(X,Y) between two (real or complex) Banach spaces X,Y is said to be a if for every T∈L(X,Y) there exists a modulus-one scalar θ such that
∥G+θT∥=1+∥T∥.
In this talk, we will give an overview of the topic, reviewing main results and ideas, and posing some open problems on the way.