The announcements are updated continuously. For a list of talks in the coming weeks, please see here.

**Vienna Research Seminar in Set Theory****Time:** Tuesday, 28 March, 15:00-16:30 CEST**Speaker:** Miguel Moreno, Universität Wien**Title:** Generalised Descriptive Set Theory, part III**Abstract:** Following part I and part II in this three part series, during this talk we will discuss where in the generalized Borel-reducibility hierarchy are the isomorphism relation of first order complete theories. These theories are divided into two kinds: classifiable and non-classifiable. To study the classifiable theories case is needed the use of Ehrenfeucht-Fraïssé games. On the other hand the study of the non-classifiable theories is done by using colored ordered trees. The goal of the talk is to see the classifiable theories case and sketch the ideas of non-classifiable theories.**Information:** This talk will be given in hybrid format. Please contact Richard Springer for information how to participate.

**Baltic Set Theory Seminar****Time:** Tuesday, 28 March, 15:00-16:30 CEST**Speaker:** Several**Title:** Baltic Set Theory Seminar**Abstract:** This is a learning seminar, the goal is to actually go over proofs and more or less understand them. Discussions are encouraged. The topic of the seminar is the following:

1. Sandra Müller, Stationary-tower-free proof of Woodin’s Sealing Theorem.

2. Matteo Viale, Generic absoluteness theorem for the omega_1 Chang model conditioned to MM^{+++}.**Information:** Please see the seminar webpage.

**CMU Logic Seminar****Time:** Tuesday, 28 March, 3:30 – 4:30pm Pittsburgh time (21:30 – 22:30 CEST) **Speaker:** Matthieu Joseph, Université Paris Saclay**Title:** Several notions of freeness for actions of countable groups**Abstract: **In this talk we will study different notions of freeness for actions of countable groups by homeomorphisms on compact spaces. Genuine freeness, freeness in the sense of (some) invariant measure and freeness in the sense of the topology will be compared, via concrete examples. We will bring out and discuss the class of allosteric groups, which is the class of groups that admit actions on compact spaces with an invariant measure that are free in the sense of the topology but non-free in the sense of the measure.**Information:** See the seminar webpage.

**Hebrew University-Bar Ilan University Set Theory seminar****Time:** Wednesday, 29 March, 13:00-15:00 Israel Time (12:00-14:00 CET)**Speaker:** Yair Hayut**Title:** Sealing Kurepa trees**Abstract:** In this talk, I’m going to describe Itamar Giron’s master thesis. Most of the results in this talk are due to him.

The main question of the thesis was whether there is a forcing notion that makes an arbitrary Kurepa tree into a non-distributive one, and how far can one go in this direction (can we get sealed Kurepa trees?).

We will start with the classical construction of a Kurepa tree in L (by Solovay). We will show that this tree is distributive in L. We will review the known constructions due to Poor and Shelah (generalized by Muller and me), of sealed Kurepa trees in L (can be generalized to canonical inner models).

Then, we will also find a forcing extension in which for every Kurepa tree, one can add a branch without collapsing cardinals. This means that even though it is easy to find non-distributive Kurepa trees, it is far less trivial to get from combinatorial assertions (such as diamond*), a sealed Kurepa tree.

Finally, I will talk about the forcing notion that “specializes” a Kurepa tree over an arbitrary model of ZFC. This is Giron’s main result, which requires the most sophisticated tools. **Information:** Contact Menachem Magidor or Omer Ben-Neria ahead of time for the zoom link.

**Vienna Logic ColloquiumTime:** Thursday, 30 March, 15:00 – 15:45 CET

**Speaker:**M. Pinsker, TU Wien

**Title:**Constraint Satisfaction Problems: algebraic and model-theoretic challenges to distinguish the easy from the hard

**Abstract:**I will give a gentle introduction to current algebraic and model-theoretic methods in the computational complexity of Constraint Satisfaction Problems (CSPs).

A CSP is a computational problem where we are given variables and constraints about them; the question is whether the variables can be assigned values such that all constraints are satisfied. Numerous natural computational problems, such as satisfiability of a given system of equations over a field, are CSPs; in fact, any computational problem is Turing-equivalent to a CSP.

Any CSP can be modeled by a relational structure, and conversely every relational structure naturally defines a CSP. In view of humanity’s continuing quest to distinguish easy from hard problems in general, and the class P (polynomial-time solvable problems, e.g. satisfiability of linear equations over a field) from the class NP (polynomial-time verifiable problems, e.g. satisfiability of a propositional formula) in particular, the question arises which mathematical properties of a relational structure make the corresponding CSP easy and which make it hard. It turns out that particular algebraic invariants of the structure often determine the borderline between different complexity classes. Hence algebraic methods, combined with concepts from model theory as well as from Ramsey theory in the case of infinite structures, yield appropriate tools to determine the computational complexity of CSPs.

**Information:**This talk will be given in hybrid format. Please contact Richard Springer for information how to participate.

**Cross-Alps Logic Seminar****Time:** Friday, 31 March, 16.00-17.00 CEST**Speaker:** L. Patey, CNRS**Title:** Canonical notions of forcing in Reverse Mathematics**Abstract:** In Reverse Mathematics, a proof of non-implication from a statement P to a statement Q consists of creating a model of P which is not a model of Q. To this end, one usually create a complicated instance I of Q, and then, build iteratively a model of P containing I while avoiding every solution to I. The difficult part consists in building solutions to instances of P which will not compute any solution to I. This is usually done by forcing. Moreover, by some empirical observation, the notion of forcing used in a separation of P from Q usually does not depend on Q. For example, constructing models of WKL is usually done by forcing with Pi^0_1 classes. This tends to show that P admits a “canonical” notion of forcing. In this talk, we provide a formal framework to discuss this intuition, and study the canonical notions of forcing associated to some important statements in Reverse Mathematics. This is a joint work with Denis Hirschfeldt.**Information:** The event will stream on the Webex platform. Please write to luca.mottoros [at] unito.it for the link to the event.

**CUNY Set Theory Seminar****Time:** Friday, 31 March, 12:15pm New York time (18:15 CET)**Speaker: **Benjamin Goodman, CUNY**Title:** Σn-correct forcing axioms**Abstract:** The standard method of producing a model of a forcing axiom from a supercompact cardinal in fact gives a model of an even stronger principle: that for every small name a and every Σ2 formula arphi such that φ(a) is forceable by and preserved under further forcing in our forcing class, there is a filter F which meets a desired collection of dense sets and also interprets a such that φ(aF) already holds. I will show how to generalize this result to formulas of higher complexity by starting with slightly stronger large cardinal assumptions, then discuss the bounded versions of these enhanced forcing axioms, their relationships to other similar principles, and their consequences.**Information:** The seminar will take place virtually. Please email Victoria Gitman (vgitman@nylogic.org) for the meeting id.

**Toronto Set Theory Seminar****Time:** Friday, 31 March, 1.30-3.00 Toronto time (19.30-21.00 CET)**Speaker:** tba **Title:** tba **Abstract:** tba **Information:** Please see http://gfs.fields.utoronto.ca/activities/22-23/set-theory-seminar.

**CUNY Logic Workshop****Time:** Friday, 31 March, 2:00 – 3:30 New York time (20:00-21:30 CET)**Speaker: **Corey Switzer, University of Vienna**Title: **tba **Abstract:** tba **Information:** The talk will take place in person. For more information, please see the seminar webpage or email Victoria Gitman.