For a list of talks in the coming weeks, see https://ests.wordpress.com/online-activities-2021.

**Caltech Logic Seminar****Time:** Monday, 19 April, 12:00 – 1:00pm Pacific time (21:00 CET)**Speaker:** Francois Le Maitre, University of Paris**Title:** A characterization of high transitivity for groups acting on trees**Abstract:** A countable group is highly transitive if it admits an embedding in the permutation group of the integers with dense image. I will present a joint work with Pierre Fima, Soyoung Moon and Yves Stalder where we show that a large class of groups acting on trees are highly transitive, which yields a characterization of high transitivity for groups admitting a minimal faithful action of general type on a tree thanks to the work of Le Boudec and Matte Bon. Our proof is new even for the free group on two generators and I will give a detailed overview in this very particular case, showing that the generic transitive action of the free group on two generators is highly transitive.**Information:** Check on the seminar webpage if the seminar will take place.

**Hebrew University-Bar Ilan University Set Theory seminar****Time:** Wednesday, 21 April, 14:00-16:00 Israel Time (13:00-15:00 CET)**Speaker:** Omer Ben Neria**Title:** Strong Prikry property for Magidor Iterations**Abstract:** In his celebrated work on the identity crisis of strongly compact cardinals, Magidor introduced a special iteration of Prikry forcings for a set of measurable cardinals, known as the Magidor iteration.The purpose of this talk is to state and prove a version of the strong Prikry Lemma for such iterations, extending a result of Fuchs for the case where the set of measurables is discrete. We will also describe several applications regarding the genericity of sequences of critical points in iterated ultrapowers.**Information:** Please check on the seminar webpage if the seminar will take place. Contact Menachem Magidor, Asaf Rinot or Omer Ben-Neria ahead of time for the zoom link.

**Münster research seminar on set theory****Time:** Wednesday, 21 April, 15:15-16:45 CET**Speaker:** tba**Title:** tba**Abstract:** tba**Information:** Please check the seminar webpage to see if the seminar takes place. Contact rds@wwu.de ahead of time in order to participate.

**Barcelona Set Theory Seminar****Time:** Wednesday, 21 April, 16:00-17:30 CET**Speaker:** Sam Roberts**Title:** Reinhardt’s potentialism**Abstract:** Reflection principles have been of interest to philosophers and mathematicians because they promise to be well-motivated additions to the standard axioms of set theory that nonetheless settle many of the questions left open by those axioms. Although William Reinhardt’s work on reflection principles has been immensely influential, some of his central ideas have remained unclear. The purpose of my talk will be to rectify this. I will start by outlining and formalising his primary contribution to the literature on reflection principles, which is a version of potentialism. I will then show that it is remarkably strong and discuss a number of criticisms.**Information:** Online. If you wish to attend, please send an email to bagaria@ub.edu asking for the link.

**KGRC Research Seminar, ViennaTime:** Thursday, 22 April, 15:00-16:30 CET

**Speaker:**Osvaldo Guzmán, Universidad Nacional Autónoma de México

**Title:**MAD families and strategically bounding forcings

**Abstract:**The notion of strategically bounding forcings is a natural game-theoretic strengthening of the bounding property for partial orders. In this talk, we will study the basic properties of strategically bounding forcings and talk about indestructibility of MAD families. The motivation for this work is the problem of Roitman.

**Information:**Talk via zoom.

**Turin-Udine logic seminar****Time:** Friday, 23 April, 16:30-18:30 CET**Speaker:** F. Loregian, Tallinn University of Technology**Title:** Functorial Semantics for Partial Theories**Abstract:** We provide a Lawvere-style definition for partial theories, extending the classical notion of equational theory by allowing partially defined operations. As in the classical case, our definition is syntactic: we use an appropriate class of string diagrams as terms. This allows for equational reasoning about the class of models defined by a partial theory. We demonstrate the expressivity of such equational theories by considering a number of examples, including partial combinatory algebras and cartesian closed categories. Moreover, despite the increase in expressivity of the syntax we retain a well-behaved notion of semantics: we show that our categories of models are precisely locally finitely presentable categories, and that free models exist.**Information:** Online on WebEx. Please see the seminar webpage.

**Toronto Set Theory Seminar****Time:** Friday, 23 April, 1.30-3pm Toronto time (19:30-21:00 CET)**Speaker:** tba**Title:** tba**Abstract:** tba**Information:** No webpage available. Email Ivan Ongay Valverde to receive the seminar announcements and for the zoom link.

**CUNY Set Theory Seminar****Time:** Friday, 23 April, 2pm New York time (20:00 CET)**Speaker:** Andres Villaveces, CUNY**Title:** Two logics, and their connections with large cardinals / Questions for BDGM: Part II**Abstract:** In the past couple of years I have been involved (joint work with Väänänen and independently with Shelah) with some logics in the vicinity of Shelah’s L1κ (a logic from 2012 that has Interpolation and a very weak notion of compactness, namely Strong Undefinability of Well-Orderings, and in some cases has a Lindström-type theorem for those two properties). Our work with Väänänen weakens the logic but keeps several properties. Our work with Shelah explores the connection with definability of AECs.

These logics seem to have additional interesting properties under the further assumption of strong compactness of a cardinal, and this brings them close to recent work of Boney, Dimopoulos, Gitman and Magidor [BDGM].

During the first lecture, I plan to describe two games and a syntax of two logics: Shelah’s L1κ and my own logic (joint work with Väänänen) L1,cκ. I will stress some of the properties of these logics, without any use of large cardinal assumptions. During the second lecture, I plan to enter rather uncharted territory. I will describe some constructions done by Shelah (mostly) under the assumption of strong compactness, but I also plan to bring these logics to a territory closer to the work of [BDGM]. This second lecture will have more conjectures, ideas, and (hopefully interesting) discussions with some of the authors of that paper.**Information:** The seminar will take place virtually. Please email Victoria Gitman (vgitman@nylogic.org) for the meeting id.