2018 Nov 21

# Analysis Seminar: Asaf Shachar (HUJI) "Regularity via minors and applications to conformal maps"

12:00pm to 1:00pm

## Location:

Room 70, Ross Building
Title:
Regularity via minors and applications to conformal maps.
Abstract:
Let f:\mathbb{R}^n \to \mathbb{R}^n be a Sobolev map; Suppose that the k-minors of df are smooth. What can we say about the regularity of f?
This question arises naturally in the context of Liouville's theorem, which states that every weakly conformal map is smooth. I will explain the connection of the minors question to the conformal regularity problem, and describe a regularity result for maps with regular minors.
2018 Oct 18

# Colloquium: Rahul Pandharipande (ETH Zürich) - Zabrodsky Lecture: Geometry of the moduli space of curves

2:30pm to 3:30pm

## Location:

Manchester Building (Hall 2), Hebrew University Jerusalem
The moduli space of curves, first appearing in the work of Riemann in the 19th century, plays an important role in geometry. After an introduction to the moduli space, I will discuss recent directions in the study of tautological classes on the moduli space following ideas and conjectures of Mumford, Faber-Zagier, and Pixton. Cohomological Field Theories (CohFTs) play an important role. The talk is about the search for a cohomology calculus for the moduli space of curves parallel to what is known for better understood geometries.
2018 Dec 06

# Colloquium: Naomi Feldheim (Bar-Ilan) - A spectral perspective on stationary signals

2:30pm to 3:30pm

## Location:

Manchester Building (Hall 2), Hebrew University Jerusalem
A random stationary signal'', more formally known as a Gaussian stationary function, is a random function f:R-->R whose distribution is invariant under real shifts (hence stationary), and whose evaluation at any finite number of points is a centered Gaussian random vector (hence Gaussian).
The mathematical study of these random functions goes back at least 75 years, with pioneering works by Kac, Rice and Wiener, who were motivated both by applications in engineering and
by analytic questions about typical'' behavior in certain classes of functions.
2019 Mar 14

# Colloquium: Alexander Bors (University of Western Australia) - Finite groups with a large automorphism orbit

2:30pm to 3:30pm

## Location:

Manchester Building (Hall 2), Hebrew University Jerusalem
Abstract: If X is an object such that the notion of an automorphism of X is defined (e.g.,
an algebraic structure, a graph, a topological space, etc.), then one can define an
equivalence relation ∼ on X via x ∼ y if and only if α(x) = y for some automorphism
α of X. The equivalence classes of ∼ are called the automorphism orbits of X.
Say that X is highly symmetric if and only if all elements of X lie in the same
automorphism orbit. Finite highly symmetric objects are studied across various
2019 May 02

# Colloquium: Jake Solomon- Pointwise mirror symmetry

2:30pm to 3:30pm

## Location:

Manchester Building (Hall 2), Hebrew University Jerusalem

Abstract: Mirror symmetry is a correspondence between symplectic geometry on a manifold M and complex geometry on a mirror manifold W. The question of why one sort of geometry on M should be reflected in another sort of geometry on the topologically distinct manifold W, and the question of how to find W given M, are a priori highly mysterious. One attempt to explain the mysteries of mirror symmetry is the SYZ conjecture, which asserts that the mirror manifold W can be realized as the moduli space of certain objects of a category associated to M.
2018 Nov 08

# Colloquium: Nathan Keller (Bar Ilan) - The junta method for hypergraphs and the Erdos-Chvatal simplex conjecture

2:30pm to 3:30pm

## Location:

Manchester Building (Hall 2), Hebrew University Jerusalem
Numerous problems in extremal hypergraph theory ask to determine the maximal size of a k-uniform hypergraph on n vertices that does not contain an 'enlarged' copy H^+ of a fixed hypergraph H. These include well-known problems such as the Erdos-Sos 'forbidding one intersection' problem and the Frankl-Furedi 'special simplex' problem.
2019 Jun 27

# Colloquium Dvoretzky lecture: Assaf Naor(Princeton) - An average John theorem

2:30pm to 3:30pm

## Location:

Manchester Building (Hall 2), Hebrew University Jerusalem

Abstract: We will prove a sharp average-case variant of a classical embedding theorem of John through the theory of nonlinear spectral gaps. We will use this theorem to provide a new answer to questions of Johnson and Lindenstrauss (1983) and Bourgain (1985) on metric dimension reduction, and explain how it leads to algorithms for approximate nearest neighbor search.
2019 Jan 03

# Colloquium: Nati Linial (HUJI) - Graph metrics

2:30pm to 3:30pm

A finite graph is automatically also a metric space, but is there any interesting geometry to speak of? In this lecture I will try to convey the idea that indeed there is very interesting geometry to explore here. I will say something on the local side of this as well as on the global aspects. The k-local profile of a big graph G is the following distribution. You sample uniformly at random k vertices in G and observe the subgraph that they span. Question - which distributions can occur? We know some of the answer but by and large it is very open.
2019 Apr 18

(All day)

2018 Oct 25

# Colloquium: Karim Adiprasito (HUJI) - Combinatorics, topology and the standard conjectures beyond positivity

2:30pm to 3:30pm

## Location:

Manchester Building (Hall 2), Hebrew University Jerusalem
Consider a simplicial complex that allows for an embedding into R^d. How many faces of dimension d/2 or higher can it have? How dense can they be?
This basic question goes back to Descartes. Using it and other rather fundamental combinatorial problems, I will motivate and introduce a version of Grothendieck's "standard conjectures" beyond positivity (which will be explored in detail in the Sunday Seminar).
All notions used will be explained in the talk (I will make an effort to be very elementary)
2019 Jun 06

# Colloquium: Ram Band (Technion) - Neumann Domains

2:30pm to 3:30pm

## Location:

Manchester Building (Hall 2), Hebrew University Jerusalem
Abstract:
The nodal set of a Laplacian eigenfunction forms a partition of the underlying manifold.
An alternative partition, based on the gradient field of the eigenfunction, is via the so called Neumann domains.
A Neumann domain of an eigenfunction is a connected component of the intersection between the stable
manifold of a certain minimum and the unstable manifold of a certain maximum.
We introduce this subject, discuss various properties of Neumann domains and
point out the similarities and differences between nodal domains and Neumann domains.
2018 Dec 20

# Colloquium: Assaf Rinot (Bar-Ilan) - Hindman’s theorem and uncountable Abelian groups

2:30pm to 3:30pm

## Location:

Manchester Building (Hall 2), Hebrew University Jerusalem
In the early 1970’s, Hindman proved a beautiful theorem in
additive Ramsey theory asserting that for any partition of the set of
natural numbers into finitely many cells, there exists some infinite set
such that all of its finite sums belong to a single cell.
In this talk, we shall address generalizations of this statement to the
realm of the uncountable. Among other things, we shall present a
negative partition relation for the real line which simultaneously
generalizes a recent theorem of Hindman, Leader and Strauss, and a
2019 Apr 04

# Colloquium: Uri Shapira (Technion) - Dynamics on hybrid homogeneous spaces

2:30pm to 3:30pm

## Location:

Manchester Building (Hall 2), Hebrew University Jerusalem
Abstract: I will discuss a collection of results about lattices and their subgroups in Euclidean space which are obtained using dynamics on homogeneous spaces. The ergodic theory of group actions on spaces obtained by quotienning a Lie group by a lattice (spaces of lattice-type) or on projective spaces are extensively studied and display distinct dynamical phenomena. Motivated by classical questions in Diophantine approximation we are led to study the ergodic theory of group actions on hybrid homogeneous spaces which are half projective and half of lattice type.
2019 May 23

# Colloquium: Yves Benoist (University of Paris-Sud) - Arithmeticity of discrete groups

2:30pm to 3:30pm

## Location:

Manchester Building (Hall 2), Hebrew University Jerusalem
By a theorem of Borel and Harish-Chandra,
an arithmetic group in a semisimple Lie group is a lattice.
Conversely, by a celebrated theorem of Margulis,
in a higher rank semisimple Lie group G
any irreducible lattice is an arithmetic group.
The aim of this lecture is to survey an
arithmeticity criterium for discrete subgroups
which are not assumed to be lattices.
This criterium, obtained with Miquel,
generalizes works of Selberg and Hee Oh
and solves a conjecture of Margulis. It says:
2018 Nov 29

# Colloquium: Chaya Keller (Technion) - Improved lower and upper bounds on the Hadwiger-Debrunner numbers

2:30pm to 3:30pm

## Location:

Manchester Building (Hall 2), Hebrew University Jerusalem
A family of sets F is said to satisfy the (p,q)-property if among any p sets in F, some q have a non-empty intersection. Hadwiger and Debrunner (1957) conjectured that for any p > q > d there exists a constant c = c_d(p,q), such that any family of compact convex sets in R^d that satisfies the (p,q)-property, can be pierced by at most c points. Helly's Theorem is equivalent to the fact that c_d(p,p)=1 (p > d).