2017
Mar
02

# Events & Seminars

2017
Nov
02

# Group actions: Remi Coulon (Rennes) - Growth gap in hyperbolic groups and amenability

10:30am to 11:30am

## Location:

hyperbolic groups and amenability

(joint work with Françoise Dal'Bo and Andrea Sambusetti)
Given a finitely generated group G acting properly on a metric space X,
the exponential growth rate of G with respect to X measures "how big"
the orbits of G are. If H is a subgroup of G, its exponential growth
rate is bounded above by the one of G. In this work we are interested in
the following question: what can we say if H and G have the same
exponential growth rate? This problem has both a combinatorial and a
geometric origin. For the combinatorial part, Grigorchuck and Cohen

2017
Apr
27

# Group actions: Yair Glasner (BGU) - On Highly transitive permutation representations of groups.

10:30am to 11:30am

## Location:

Ross 70

Abstract: A permutation representation of a group G is called highly transitive if it is transitive on k-tuples of points for every k. Until just a few years ago groups admitting such permutation representations were thought of as rare. I will focus on three rather recent papers: G-Garion, Hall-Osin, Gelander-G-Meiri (in preparation) showing that such groups are in fact very common.

2018
May
10

# Groups & dynamics: Sanghoon Kwon (Kwandong University) - A combinatorial approach to the Littlewood conjecture in positive characteristic

10:30am to 11:30am

## Location:

Ross 70

The Littlewood conjecture is an open problem in simultaneous Diophantine approximation of two real numbers. Similar problem in a field K of formal series over finite fields is also still open. This positive characteristic version of problem is equivalent to whether there is a certain bounded orbit of diagonal semigroup action on Bruhat-Tits building of PGL(3,K).

2018
Feb
19

2018
Jan
21

2017
Jan
05

2017
Jan
19

2017
Nov
28

# T&G: Benjamin Ackermann (Hebrew University), Kodaira's embedding theorem

12:00pm to 1:30pm

## Location:

Room 70A, Ross Building, Jerusalem, Israel

In this talk we present a proof of the Kodaira's theorem that gives a sufficient condition on the existence of an embedding of a Kahler manifold into CPn. This proof is based on the Kodaira Vanishing theorem, using a sheaf-cohomological translation of the embedding conditions.
לאירוע הזה יש שיחת וידאו.
הצטרף: https://meet.google.com/mcs-bwxr-iza

2015
Nov
09

# Number theory: Ishai Dan-Cohen (Essen), "Towards Chabauty-Kim loci for the polylogarithmic quotient over an arbitrary number field"

4:00pm to 5:45pm

## Location:

Ross Building, room 70, Jerusalem, Israel

Title: Towards Chabauty-Kim loci for the polylogarithmic quotient over an arbitrary number field
Abstract: Let K be a number field and let S be an open
subscheme of Spec O_K.
Minhyong Kim has developed a method for
bounding the set of S-valued points on a
hyperbolic curve X over S; his method opens
a new avenue in the quest for an "effective
Mordell conjecture".
But although Kim's approach has lead to the
construction of explicit bounds in special
cases, the problem of realizing the potential
effectivity of his methods remains a difficult
and beautiful open problem.

2015
Dec
07

# Number theory: Jean-Baptiste Teyssier (HUJI) "Kedlaya-Mochizuki theorem and applications"

4:00pm to 5:15pm

## Location:

Ross Building, room 70A

Let X be a complex manifold and let M be a meromorphic connection on X with
poles along a normal crossing divisor D. Levelt-Turrittin theorem asserts that the pull-back of M to the formal neighbourhood of a codimension 1 point in D decom poses (after ramification) into elementary factors easy to work with.
This decomposition may not hold at some other points of D. When it does, we say
that M has good formal decomposition along D. A conjecture of Sabbah, recently
proved by Kedlaya and Mochizuki independently, asserts roughly the

2017
Dec
19

# T&G: Yakov Eliashberg (Stanford), Simplifying singularities of Lagrangian skeleta

1:00pm to 2:30pm

## Location:

Room 63, Ross Building, Jerusalem, Israel

I will discuss in the talk David Nadler’s “arborealizaton conjecture” and will sketch its proof. The conjecture states that singularities of a Lagrangian skeleton of a symplectic Weinstein manifold could be always simplified to a finite list of singularities, called ``arboreal”. This is a joint work with Daniel Albarez-Gavela, David Nadler and Laura Starkston.

2016
Jun
02

# Groups & dynamics: Todor Tsankov (Paris-Diderot): On metrizable universal minimal flows

10:00am to 11:00am

## Location:

Ross building, Hebrew University of Jerusalem, (Room 70)

To every topological group, one can associate a unique universal
minimal flow (UMF): a flow that maps onto every minimal flow of the
group. For some groups (for example, the locally compact ones), this
flow is not metrizable and does not admit a concrete description.
However, for many "large" Polish groups, the UMF is metrizable, can be
computed, and carries interesting combinatorial information. The talk
will concentrate on some new results that give a characterization of
metrizable UMFs of Polish groups. It is based on two papers, one joint

2016
Mar
29

2015
Dec
17

# Groups & dynamics: Rene Rühr, Distribution of Shapes of Orthogonal Lattices

10:00am to 11:30am

## Location:

Ross building, Hebrew University of Jerusalem, (Room 70)

To every topological group, one can associate a unique universal
minimal flow (UMF): a flow that maps onto every minimal flow of the
group. For some groups (for example, the locally compact ones), this
flow is not metrizable and does not admit a concrete description.
However, for many "large" Polish groups, the UMF is metrizable, can be
computed, and carries interesting combinatorial information. The talk
will concentrate on some new results that give a characterization of
metrizable UMFs of Polish groups. It is based on two papers, one joint