Events & Seminars

2017 May 29

Logic seminar - Ur Yaar, "A Toy Multiverse"

2:00pm to 4:00pm

Location: 

Shprinzak 101
We will present briefly the "multiverse view" of set theory, advocated by Hamkins, that there are a multitude of set-theoretic universes, and not one background universe, and his proposed "Multiverse Axioms". We will then move on to present the main result of Gitman and Hamkins in their paper "A natural model of the multiverse axioms" - that the countable computably saturated models of ZFC form a "toy model" of the multiverse axioms.
2017 Nov 22

Logic Seminar - Yair Hayut - "Chang's Conjecture at many cardinals simultaneously"

11:00am to 1:00pm

Location: 

Math 209
Chang's Conjecture is a strengthening of Lowenheim-Skolem-Tarski theorem. While Lowenheim-Skolem-Tarski theorem is provable in ZFC, any instance of Chang's Conjecture is independent with ZFC and has nontrivial consistency strength. Thus, the question of how many instances of Chang's Conjecture can consistently hold simultaneously is natural.

I will talk about some classical results on the impossibility of some instances of Chang's Conjecture and present some results from a joint work with Monroe Eskew.

 
2016 Dec 27

Special logic seminar - Itaï BEN YAACOV, "Baby version of the asymptotic volume estimate"

10:00am to 12:00pm

Location: 

Shprinzak 102
I'll show how the Vandermonde determinant identity allows us to estimate the volume of certain spaces of polynomials in one variable (or rather, of homogeneous polynomials in two variables), as the degree goes to infinity. I'll explain what this is good for in the context of globally valued fields, and, given time constraints, may give some indications on the approach for the "real inequality" in higher projective dimension.
2017 Dec 27

Logic Seminar - Omer Ben-Neria - "Singular Stationarity and Set Theoretic Generalizations of Algebras"

11:00am to 1:00pm

Location: 

Ross 63
Abstract: The set theoretic generalizations of algebras have been introduced in the 1960s to give a set theoretic interpretation of usual algebraic structures. The shift in perspective from algebra to set theory is that in set theory the focus is on the collection of possible algebras and sub-algebras on specific cardinals rather than on particular algebraic structures. The study of collections of algebras and sub-algebras has generated many well-known problems in combinatorial set theory (e.g., Chang’s conjecture and the existence of small singular Jonsson cardinals).
2017 Nov 01

Logic Seminar - Immanuel BenPorat - "Cardinal conditions for strong Fubini theorems"

11:00am to 1:00pm

Location: 

Math209
This talk will be largely based on a paper by Joseph Shipman with the same title. We will discuss some variations of Fubini type theorems. The focus will be on what is known as "strong Fubini type theorems". Apparently these versions were proved to be independent of ZFC,and our main aim will be to sketch a proof of this result. We will assume basic knowledge in measure theory. Aside from that, the material is rather self contained.
2017 Dec 07

Combinatorics: Shira Zerbib Gelaki (MSRI, U. Michigan) "Colorful coverings of polytopes -- the hidden topological truth behind different colorful phenomena"

12:00pm to 1:00pm

Location: 

Room 101 in Sprinzak
Speaker: Shira Zerbib Gelaki (MSRI, University of Michigan) Title: Colorful coverings of polytopes -- the hidden topological truth behind different colorful phenomena Abstract: The topological KKMS Theorem is a powerful extension of the Brouwer's Fixed-Point Theorem, which was proved by Shapley in 1973 in the context of game theory. We prove a colorful and polytopal generalization of the KKMS Theorem, and show that our theorem implies some seemingly unrelated results in discrete geometry and combinatorics involving colorful settings.
2017 Feb 27

NT&AG: Eyal Goren (McGill University), "p-adic dynamics of Hecke operators"

3:00pm to 4:00pm

Location: 

Ross 70A
Abstract: Motivated by understanding the action of Hecke operators on special sub-varieties of Shimura varieties, we consider the simplest possible case: the action of Hecke operators on the j-line, namely on the moduli space of elliptic curves, and in particular the action on singular moduli. Our interest is in this action considered in the p-adic topology. The emerging picture is surprisingly rich and the answers involve Serre-Tate coordinates, the Gross-Hopkins period map and finally involves random walks on GL_n. This is joint work with Payman Kassaei (King's College).
2017 Dec 25

NG&AT: Zev Rosengarten (Stanford University), "Tamagawa Numbers of Linear Algebraic Groups Over Function Fields"

2:00pm to 3:00pm

Location: 

Room 70A, Ross Building, Jerusalem, Israel
Abstract: In 1981, Sansuc obtained a formula for Tamagawa numbers of reductive groups over number fields, modulo some then unknown results on the arithmetic of simply connected groups which have since been proven, particularly Weil's conjecture on Tamagawa numbers over number fields. One easily deduces that this same formula holds for all linear algebraic groups over number fields. Sansuc's method still works to treat reductive groups in the function field setting, thanks to the recent resolution of Weil's conjecture in the function field setting by Lurie and Gaitsgory.
2016 Jun 02

Number theory: Eran Asaf (HUJI) "Invariant norms in representations of GL_2(Q_p)"

12:00pm to 1:15pm

Location: 

Hebrew University, Givat Ram, Ross Building, room 63
A natural question is whether there exists a continuous p-adic analogue for the classical local Langlands correspondence for GL_n(F) . Namely, for a finite extension F of Q_p, we want to associate continuous p -adic representations of GL_n(F) to n-dimensional p-adic representations of the Weil group of F. The particular case, where F=Q_p and n=2 , is now known. One of the main tools for establishing this correspondence was the existence of GL_2(Q_p)-invariant norms in certain representations of GL_2(Q_p).
2017 Jan 02

NT&AG: Ehud de Shalit (HUJI), "Geometry modulo p of some unitary Shimura varieties"

2:00pm to 3:00pm

Location: 

Ros Building, 70A
Abstract: This talk will be about joint work with Eyal Goren about the structure of Picard modular surfaces at a prime p which is inert in the underlying quadratic imaginary field. The main tool for studying the bad reduction of Shimura varieties is the theory of local models (due to de Jong and Rapoport-Zink). Our results concern global geometric questions which go beyond the theory of global models. For example, we are able to count supersingular curves on the Picard surface. We also study certain foliations in its tangent bundle that have not been studied before, and
2016 Dec 05

NT&AG: Michael Temkin (Hebrew University), "Topological transcendence degree"

2:00pm to 3:00pm

Abstract: my talk will be devoted to a basic theory of extensions of complete real-valued fields L/K. Naturally, one says that L is topologically-algebraically generated over K by a subset S if L lies in the completion of the algebraic closure of K(S). One can then define topological analogues of algebraic independence, transcendence degree, etc. These notions behave much more wierd than their algebraic analogues. For example, there exist non-invertible continuous K-endomorphisms of the completed algebraic closure of K(x). In my talk, I will tell which part
2017 Jun 19

NT&AG: Ehud de Shalit (HUJI) "Ordinary foliations on unitary Shimura varieties"

2:00pm to 3:00pm

Abstract: Inseparable morphisms proved to be an important tool for the study of algebraic varieties in characteristic p. In particular, Rudakov-Shafarevitch, Miyaoka and Ekedahl have constructed a dictionary between "height 1" foliations in the tangent bundle and "height 1" purely inseparable quotients of a non-singular variety in characteristic p. In a joint work with Eyal Goren we use this dictionary to study the special fiber S of a unitary Shimura variety of signature (n,m), m < n, at a prime p which is inert in the underlying imaginary quadratic field. We

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