Events & Seminars

2017 Jun 01

Group actions:Lei Yang - badly approximable points on curves and unipotent orbits in homogeneous spaces

10:30am to 11:30am

We will study n-dimensional badly approximable points on curves. Given an analytic non-degenerate curve in R^n, we will show that any countable intersection of the sets of weighted badly approximable points on the curve has full Hausdorff dimension. This strengthens a previous result of Beresnevich by removing the condition on weights. Compared with the work of Beresnevich, we study the problem through homogeneous dynamics. It turns out that the problem is closely related to the study of distribution of long pieces of unipotent orbits in homogeneous spaces.
2016 Mar 17

Colloquium-Landau Lectures: Ravi Vakil (Stanford) "Cutting and pasting in algebraic geometry, and the motivic zeta function"

3:30pm to 4:30pm

Location: 

Manchester Building (Hall 2), Hebrew University Jerusalem
Given some class of "geometric spaces", we can make a ring as follows. 1. (additive structure) When U is an open subset of such a space X, [X] = [U] + [(X \ U)] 2. (multiplicative structure)} [X x Y] = [X] [Y]. In the algebraic setting, this ring (the "Grothendieck ring of varieties") contains surprising structure, connecting geometry to arithmetic and topology.
2015 Nov 26

Colloquium: Shai Evra (HUJI), "Topological Expanders"

2:30pm to 3:30pm

Location: 

Manchester Building (Hall 2), Hebrew University Jerusalem
Title: Topological Expanders. Abstract: A classical result of Boros-Furedi (for d=2) and Barany (for d>=2) from the 80's, asserts that given any n points in R^d, there exists a point in R^d which is covered by a constant fraction (independent of n) of all the geometric (=affine) d-simplices defined by the n points. In 2010, Gromov strengthen this result, by allowing to take topological d-simplices as well, i.e. drawing continuous lines between the n points, rather then straight lines and similarly continuous simplices rather than affine.
2016 Dec 29

Colloquium: Jordan Ellenberg (University of Wisconsin) "The cap set problem"

2:30pm to 3:30pm

Location: 

Manchester Building (Hall 2), Hebrew University Jerusalem
A very old question in additive number theory is: how large can a subset of Z/NZ be which contains no three-term arithmetic progression? An only slightly younger problem is: how large can a subset of (Z/3Z)^n be which contains no three-term arithmetic progression? The second problem was essentially solved in 2016, by the combined work of a large group of researchers around the world, touched off by a brilliantly simple new idea of Croot, Lev, and Pach.
2016 Mar 03

Colloquium: Sara Tukachinsky (Hebrew University) "Counts of holomorphic disks by means of bounding chains"

3:30pm to 4:30pm

Location: 

Manchester Building (Hall 2), Hebrew University Jerusalem
Over a decade ago, Welschinger defined invariants of real symplectic manifolds of complex dimensions 2 and 3, which count pseudo-holomorphic disks with boundary and interior point constraints. Since then, the problem of extending the definition to higher dimensions has attracted much attention.
2015 Nov 12

Colloquium: Michael Krivelevich (Tel Aviv), "Positional games"

2:30pm to 3:30pm

Location: 

Manchester Building (Hall 2), Hebrew University Jerusalem
Title: Positional games Positional games are a branch of combinatorics, researching a variety of two-player games, ranging from popular recreational games such as Tic-Tac-Toe and Hex, to purely abstract games played on graphs and hypergraphs. It is closely connected to many other combinatorial disciplines such as Ramsey theory, extremal graph and set theory, probabilistic combinatorics, and to computer science.
2016 May 05

Colloquium: Daniel Wise (McGill) "The Cubical Route to Understanding Groups"

2:30pm to 3:30pm

Location: 

Manchester Building (Hall 2), Hebrew University Jerusalem
Abstract: Cube complexes have come to play an increasingly central role within geometric group theory, as their connection to right-angled Artin groups provides a powerful combinatorial bridge between geometry and algebra. This talk will introduce nonpositively curved cube complexes, and then describe the developments that have recently culminated in the resolution of the virtual Haken conjecture for 3-manifolds, and simultaneously dramatically extended our understanding of many infinite groups.
2016 Dec 15

Colloquium: Cy Maor (Toronto) "Asymptotic rigidity of manifolds"

2:30pm to 3:30pm

Location: 

Manchester Building (Hall 2), Hebrew University Jerusalem
Liouville's rigidity theorem (1850) states that a map $f:\Omega\subset R^d \to R^d$ that satisfies $Df \in SO(d)$ is an affine map. Reshetnyak (1967) generalized this result and showed that if a sequence $f_n$ satisfies $Df_n \to SO(d)$ in $L^p$, then $f_n$ converges to an affine map. In this talk I will discuss generalizations of these theorems to mappings between manifolds and sketch the main ideas of the proof (using techniques from the calculus of variations and from harmonic analysis). Finally, I will describe how these rigidity questions are related to weak
2016 Jan 07

Colloquium: Peter Ozsváth (Princeton), "Zabrodsky Lectures: Knot Floer homology"

3:30pm to 4:30pm

Location: 

Manchester Building (Hall 2), Hebrew University Jerusalem
Abstract: Knot Floer homology is an invariant for knots, defined using methods from symplectic geometry. This invariant contains topological information about the knot, such as its Seifert genus; it can be used to give bounds on the unknotting number; and it can be used to shed light on the structure of the knot concordance group. I will outline the construction and basic properties of knot Floer. Knot Floer homology was originally defined in collaboration with Zoltan Szabo, and independently by Jacob Rasmussen.
2016 May 26

Colloquium: John Lott (Berkeley) "3D Ricci flow since Perelman"

2:30pm to 3:30pm

Location: 

Manchester Building (Hall 2), Hebrew University Jerusalem
I’ll talk about the advances and open questions in three dimensional Ricci flow. Topics include the finiteness of the number of surgeries, the long-time behavior and flowing through singularities. No prior knowledge of Ricci flow will be assumed.
2015 Dec 24

Colloquium: Yakov Eliashberg (Stanford) ״Crossroads of symplectic rigidity and flexibility״

2:30pm to 3:30pm

Location: 

Manchester Building (Hall 2), Hebrew University Jerusalem
Abstract: The development of flexible and rigid sides of symplectic and contact topology towards each other shaped this subject since its inception, and continues shaping its modern development. In the talk I will discuss the history of this struggle and describe recent breakthroughs on the flexible side.
2016 Dec 01

Colloquium: Shaul Zemel (Hebrew University) "Actions of Groups on Compact Riemann Surfaces"

2:30pm to 3:30pm

Location: 

Manchester Building (Hall 2), Hebrew University Jerusalem
A compact Riemann surface gives rise to several families of vector spaces, associated to divisors on the Riemann surface. A finite group G of automorphisms acts on the spaces associated with invariant divisors, and a natural question is to characterize the resulting representations of G. We show how a very simple normalization for the invariant divisors can help in answering this question in a very direct manner, and if time permits present some applications.
2015 Dec 03

Colloquium: Ofer Zeitouni (Weizmann), "Extremes of logarithmically correlated fields"

2:30pm to 3:30pm

Location: 

Manchester Building (Hall 2), Hebrew University Jerusalem
Title: Extremes of logarithmically correlated fields Abstract: The general theory of Gaussian processes gives a recipe for estimating the maximum of a random field, which is neither easy to compute nor sharp enough for obtaining the law of the maximum. In recent years, much effort was invested in understanding the extrema of logarithmically correlated fields, both Gaussian and non-Gaussian. I will explain the motivation, and discuss some of the recent results and the techniques that have been involved in proving them.

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