2019
May
15

# Tzafriri lecture: Amir Algom - A simultaneous version of Host's equidistribution Theorem

4:00pm to 5:00pm

## Location:

Manchester Building (Hall 2), Hebrew University Jerusalem

Abstract:

2019
May
15

4:00pm to 5:00pm

Manchester Building (Hall 2), Hebrew University Jerusalem

Abstract:

2018
Oct
18

2:30pm to 3:30pm

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

2:30pm to 3:30pm

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
May
02

2:30pm to 3:30pm

Manchester Building (Hall 2), Hebrew University Jerusalem

2018
Nov
08

2:30pm to 3:30pm

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

2:30pm to 3:30pm

Manchester Building (Hall 2), Hebrew University Jerusalem

2019
Jan
03

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

2:30pm to 3:30pm

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

2:30pm to 3:30pm

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

2:30pm to 3:30pm

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
classic theorem of Galvin and Shelah.

2019
Apr
04

2:30pm to 3:30pm

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

2:30pm to 3:30pm

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:
a discrete irreducible Zariski-dense subgroup
of G that intersects cocompactly at least one

2018
Nov
29

2:30pm to 3:30pm

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).

2019
Mar
21

(All day)