Organizers

prof. dr hab. Witold Marciszewski
prof. dr hab. Piotr Zakrzewski

Information

Wednesdays, 4:15 p.m. , room: 5050

Research fields

Topology and Set Theory

List of talks

June 14, 2023, 4:15 p.m.
Jakub Andruszkiewicz (Doctoral School of Exact and Natural Sciences UW)
Cofinalities of the symmetric group and of the ultrapowers of ω
We consider the following cardinal invariants of the continuum: cf(Sym(ω)), being the cofinality of the group of permutations of ω, and mcf, defined as the minimal cofinality of an ultrapower of ω over any non-principal …
May 31, 2023, 4:15 p.m.
Piotr Koszmider (IM PAN)
Ramsey properties of the distance on nonseparable spheres
Given a nonseparable metric space (M,d) bounded by 2 we consider (A) dichotomies D_r for r in (0,2): either there is uncountable subset N of M such that d(x, y) > r for all distinct …
May 24, 2023, 4:15 p.m.
Grzegorz Plebanek (University of Wrocław)
Countable discrete extensions of compact lines
We consider a separable compact line K and its extension L consisting of K and a countable number of isolated points. The main object of study is the existence of a bounded extension operator E: …
May 10, 2023, 4:15 p.m.
Aristotelis Panagiotopoulos (Carnegie Mellon University)
Menger continua via projective Fraïssé theory
Projective Fraïssé theory was introduced by T.Irwin and S.Solecki as a natural framework for analyzing the dynamics of homeomorphism groups of compact metrizable spaces in terms of finite combinatorics. In this talk I will provide …
April 26, 2023, 4:15 p.m.
Jarosław Swaczyna (Lodz University of Technology)
Continuity of coordinate functionals for ideal Schauder basis
Given an ideal of subsets of natural numbers I we say that a sequence (x_n) is I-convergent to x if for every ε>0 condition {n \in N:d(x_n,x)>ε}\in I holds. We may use this notion to …
April 19, 2023, 4:15 p.m.
Tomasz Weiss (Cardinal Stefan Wyszyński University in Warsaw)
Countably perfectly meager and countably perfectly null sets
We study a strengthening of the notion of a universally meager set and its dual counterpart that strengthens the notion of a universally null set. We say that a subset A of a perfect Polish …
March 29, 2023, 4:15 p.m.
Daria Michalik (University of Warsaw)
Blocking properties of the diagonal in Cartesian product
The abstract of the talk can be found on the webpage of our seminar: https://www.mimuw.edu.pl/en/seminaria/topology-and-set-theory-seminar
- abstract
March 22, 2023, 4:15 p.m.
Taras Banakh (Ivan Franko National University of Lviv and UJK Kielce)
An example of a 36-Shelah group
A group $G$ is called $n$-Shelah if $G=A^n$ for any subset $A\subseteq G$ of cardinality $|A|=|G|$. In 1980 Saharon Shelah constructed his famous CH-example of an uncountable 6640-Shelah group. This group was the first example …
March 15, 2023, 4:15 p.m.
Wiesław Kubiś (Akademia Nauk Republiki Czeskiej)
Ultrametric homogeneous structures
We shall present the theory of homogeneous Polish ultrametric structures. Our starting point is a Fraı̈ssé class of finite structures and the crucial tool is the universal homogeneous epimorphism. The new Fraı̈ssé limit is an …
March 8, 2023, 4:15 p.m.
Zdeněk Silber (IM PAN)
Weak* derived sets
The weak* derived set of a subset A of a dual Banach space X* is the set of weak* limits of bounded nets in A. It is known that a convex subset of a dual …
March 1, 2023, 4:15 p.m.
Adam Kwela (University of Gdańsk)
Katětov order and its applications
This talk is an overview of my recent articles on ideals on countable sets. I will present set-theoretic and topological applications of Katětov order on ideals, focusing on distinguishing certain classes of sequentially compact spaces …
Jan. 25, 2023, 4:15 p.m.
Kamil Ryduchowski (Doctoral School of Exact and Natural Sciences UW)
On antiramsey colorings of uncountable squares and geometry of nonseparable Banach spaces
A subset Z of a Banach space X is said to be r-equilateral (r-separated) if every two distinct elements of Z are in the distance exactly (at least) r from each other. We will address …
Jan. 18, 2023, 4:15 p.m.
Piotr Szewczak (Cardinal Stefan Wyszyński University in Warsaw)
Totally imperfect Menger sets: Part 2
A set of reals X is Menger if for any countable sequence of open covers of X one can pick finitely many elements from every cover in the sequence such that the chosen sets cover …
Jan. 11, 2023, 4:15 p.m.
Piotr Szewczak (Cardinal Stefan Wyszyński University in Warsaw)
Totally imperfect Menger sets
A set of reals X is Menger if for any countable sequence of open covers of X one can pick finitely many elements from every cover in the sequence such that the chosen sets cover …
Dec. 21, 2022, 4:15 p.m.
Damian Sobota (Universität Wien, Kurt Gödel Research Center for Mathematical Logic)
On continuous operators from Banach spaces of Lipschitz functions onto c_0
During my talk I will discuss some of our recent results concerning the existence of continuous operators from the Banach spaces Lip_0(M) of Lipschitz real-valued functions on metric spaces M onto the Banach space c_0 …