2015 Dec 10

# Colloquium: Tomer Schlank (HUJI), "Ultra-Products and Chromatic Homotopy Theory."

2:30pm to 3:30pm

## Location:

Manchester Building (Hall 2), Hebrew University Jerusalem
Title: Ultra-Products and Chromatic Homotopy Theory. Abstract: The category of spectra is one of the most important constructions in modern algebraic topology,
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.
2016 Nov 10

# Colloquium: Peter Sarnak (IAS & Princeton) "Navigating PU(2) ,Golden Gates and Strong Approximation"

2:30pm to 3:30pm

## Location:

Manchester Building (Hall 2), Hebrew University Jerusalem
We discuss recent developments concerning ”Golden Gates” which are number theoretic generators of PU(2) ,their application to the construction of optimally efficient universal quantum gates ,and some closely connected questions of complexity in strong approximation.
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.
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.
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 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.
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 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 Mar 31

# Colloquium: Ronen Eldan (Weizmann) "Interplays between stochastic calculus and geometric inequalities."

2:30pm to 3:30pm

## Location:

Manchester Building (Hall 2), Hebrew University Jerusalem
Abstract:
2016 Nov 17

# Colloquium: Boris Zilber (Oxford) " A model-theoretic semantics of algebraic quantum mechanics"

2:30pm to 3:30pm

## Location:

Manchester Building (Hall 2), Hebrew University Jerusalem
We approach the formalism of quantum mechanics from the logician point of view and treat the canonical commutation relations and the conventional calculus based on it as an algebraic syntax of quantum mechanics. We then aim to establish a geometric semantics of this syntax. This leads us to a geometric model, the space of states with the action of time evolution operators, which is a limit of finite models. The finitary nature of the space allows us to give a precise meaning and calculate various classical quantum mechanical quantities.