Previous talks this semester:
07.02.2019. 10.15, UNUSUAL ROOM 403;
Alessandro Vignati (KU Leuven)
Uniform Roe coronas
Joint session with the Geometric Group Theory Seminar
Given a metric space (X,d), one defines a subalgebra of the space of operators on l2(X) called the uniform Roe algebra of (X,d), denoted Cu*(X). This is the closure of the algebra of finite-propagation operators. The study of these algebras comes from the fact that Cu*(X) catches algebraically some of the large scale geometrical properties of X. Uniform Roe algebras have therefore an intrinsic relation with coarse geometry and the coarse Baum-Connes conjecture.
In recent year, much work was dedicated to show which geometric properties are preserved by isomorphisms of Uniform Roe algebras. Namely, if Cu*(X) and Cu*(Y) are isomorphic, how much do X and Y look alike? We pose the same question for Uniform Roe corona algebras.
Since Cu*(X) contains all compact operators, we can define the natural quotient
Qu*(X)=Cu*(X)/K(l2(X)), the Uniform Roe corona algebra of X. Which geometric properties do the spaces X and Y share, when an isomorphism between Qu*(X) and Qu*(Y) is given? For example, must X and Y be coarsely equivalent, or even bijectively coarsely equivalent? (Two spaces are coarsely equivalent if "they look the same when the observer is far from them").
We answer these questions with the aid of some set theory, in particular of Forcing Axioms. Forcing Axioms are generalizations of the Baire category theorem. They are alternative to the Continuum Hypothesis, and they're at the base of many rigidity phenomena observed in the theory of quotients (both discrete such as Boolean algebra quotient, and continuous, as the Calkin algebra or corona C*-algebras).
The talk starts with introducing the objects in play. The goal is to state the main results, and at least sketch the salient points of their proofs. We conclude with a list of open questions. This is joint work with Bruno Braga and Ilijas Farah.
31.01.2019. 10.15, room 105;
Arturo Martínez-Celis (IM PAN)
Porous sets on the Cantor set
Given a completely metrizable space X, a subset A of X is a strongly porous set if there is a positive constant p such that for any open ball B of radius r smaller than 1, there is an open ball B' inside of B of radius rp such that B' evades the set A. We will study the cardinal invariants related to the σ-ideal generated by strongly porous sets on the Cantor space and its relation with other known σ-ideals of the real line. We will also uncover a deep connection between the σ-ideal of the strongly porous sets and some instances of the Martin's Axiom.
17.01.2019. 10.15, room 105;
Accessible points, harmonic measure and the Riemann mapping
Let D be a bounded domain in Rn, n larger than 1. We provide an elementary proof that the set of all boundary accessible points of D is an analytic set. We investigate the nature of the set of accessible points of D when n=2 using only set theoretical methods. We provide also a view of the relation between harmonic measure in D, if n=2, D simply connected, and the Riemann mapping of D. In this talk we prove new results and give easier proofs of known results.
10.01.2019. 10.15, room 105;
Rosenthal's lemma and its applications
In this instructional talk we will recall Rosenthal's lemma on uniformly bounded sequences of measures and present its several classical applications in the Banach space and vector measures theory. First, we will prove the surprising Nikodym's uniform boundedness principle and Phillips' lemma where the application of Rosenthal's result makes the proofs much easier than the original ones. A few further corollaries of Nikodym's principle will be mentioned, such as the Dieudonné-Grothendieck theorem on bounded vector measures and the Seever theorem on the range of an operator into a B(Σ)-space. Next, we shall prove two beautiful consequences of Rosenthal's lemma: the Diestel-Faires theorem and the Orlicz-Pettis theorem. If time allows, we will also briefly discuss their further deep consequences in the structural theory of Banach spaces.
20.12.2018. 10.15, room 105;
(Kurt Godel RC, Vienna)
The Josefson--Nissenzweig theorem for Cp(X)-spaces
The famous Josefson--Nissenzweig theorem asserts that for every
infinite-dimensional Banach space X there exists a sequence (xn*) in
the dual space X* which is weak* convergent to 0 and each xn* has
norm 1. Despite the apparent simplicity of the theorem no constructive
proof --- even in the case of Banach spaces of continuous functions on
compact spaces --- has been known.
Recently, Banakh, Śliwa and Kąkol in their studies of separable
quotients of topological vector spaces of the form Cp(X), i.e. spaces
of continuous functions on Tychonoff spaces endowed with the pointwise
convergence topology, have obtained several results characterizing
those Cp(X)-spaces for which the Josefson--Nissenzweig theorem holds.
During my talk I will present some introductory facts concerning the
theorem for Cp(X)-spaces, show that the existence of
"Josefson--Nissenzweig" sequences for Cp(K)-spaces, where K is
compact Hausdorff, is strongly related to a variant of the
Grothendieck property of Banach spaces, as well as prove that every
compact space obtained as a limit of an inverse system consisting only
of minimal extensions admits such sequences (and the proof is
This is a joint work with Lyubomyr Zdomskyy.
13.12.2018. 10.15, room 105;
Controlling linear operators on C(K)s through the rigidity of K.
In the second talk of the series devoted to classical phenomena in Banach spaces of the form C(K) we will see how
linear operators on a C(K) can be represented by continuous maps from K into the space of the Radon measures
on K with the weak* topology. Previously presented results concerning weakly compact subsets in M(K) will allow us
to obtain a "geometric" understading of
the rigidity conditions on the algebra of all linear operators (having few operators modulo weakly compact operators) as
versions of topological or Boolean rigidity conditions (having few continuous maps or few Boolean endomorphisms) which
can be imposed on K.
29.11.2018. 10.15, room 105;
The Grothendieck property for Banach spaces of continuous functions
In the first talk of the series devoted to classical phenomena in Banach spaces of the form C(K) we will see how
weakly compact sets in the dual space to C(K) generalize finite subsets of K. The concrete goal will be to
motivate the Grothendieck property (weak and weak* convergence coincide in the dual) for C(K)s as a generalization of K having no nontrivial convergent sequence
and to prove that l∞≡C(βN) has the Grothendieck property.
This will require the proof of the Grothendieck-Dieudonne characterization of weakly compact sets in the spaces of measures.
All the results and proofs presented during the talk are in classical texbooks, but we will try to represent combinatorial and topological bias, leading in the following talks to more set-theoreic issues. The purpose of this series of talks is to introduce
particpants with the set-theoretic topological background to some topics related to Banach spaces of the form C(K). The area is
quite sensitive to infinitary combinatorics, e.g.,
Talagrand: CH implies that there is infinite K such that C(K) is Grothendieck but does not have l∞ as its quotient;
Haydon, Levy, Odell: p=2ω>ω1 implies that every Grothendieck C(K) for K infinite has
l∞ as its quotient;
22.11.2018. 10.15, room 105;
(IM PAN / MIM UW)
Bases of Banach spaces with respect to filters.
In 2011, Vladimir Kadets proposed the following problem: Given a filter F of subsets of natural numbers and a Banach space X, we say that a sequence (en) in X forms an F-basis, provided that every x in X has a unique representation as a series of linear combinations of en's, where the convergence is understood in the norm topology and with respect to F. Thus, for F being the filter of cofinite sets we obtain the classical notion of Schauder basis for which it is well-known that all the coordinate functionals are automatically continuous. The question is whether, they must be continuous for a general filter F. I shall present a positive answer to this questions in the case where the character of F is smaller than the pseudointersection number (published in Studia Math. 2012). Unfortunately, the answer is still not known in the important case where F is the statistical filter consisting of all sets of asymptotic density 1. We will also discuss some other related open problems concerning bases with brackets and with individual brackets.
15.11.2018. 10.15, room 105;
A capturing construction scheme from the diamond principle. Continuation from 8.11.18.
08.11.2018. 10.15, room 105;
A capturing construction scheme from the diamond principle.
S. Todorcevic introduced the concept of a capturing construction scheme
and showed it is consistent with the diamond principle. A construction scheme is a well-founded
family of finite subsets of ω1. We give a quick presentation of
the history and motivation for this tool and show that it follows from the diamond principle.
25.10.2018. 10.15, room 105;
Arturo Martínez-Celis (IM PAN)
On the Michael Space Problem
A Lindelof Topological space is Michael if it has non-Lindelof product with the space of the irrational numbers. These kind of spaces were introduced by Ernest Michael in 1963 and it is still unknown if one can be constructed in ZFC. We will introduce the notion of Michael ultrafilter, which implies the existence of a Michael space. We will also discuss the relation between this kind of ultrafilters and some classical cardinal invariants and we will use this to study the behaviour of this notion in some models of set theory.
11.10.2018. 10.15, room 105;
Piotr Koszmider (IMPAN)
Uncountable constructions from CH using generic filters
We will present some old CH constructions due to S. Shelah. As usual they use
transfinie induction, diagonalization and enumeration of all relevant objects
in type ω1. However, the use of the Martin's axiom type arguments
makes them additionally powerful.
Talks in the second semester of 2017-18.
Talks in the first semester of 2017-18.
Talks in the second semester of 2016-17.
Talks in the first semester of 2016-17.
Talks in the second semester of 2015-16.
Talks in the first semester of 2015-16.
Talks in the second semester of 2014-15.
Talks in the first semester of 2014-15.
Talks in the second semester of 2013-14.
Talks in the first semester of 2013-14.
Talks in the second semester of 2012-13.
Talks in the first semester of 2012-13.
Talks in the second semester of 2011-12.
Talks in the first semester of 2011-12.