2017
Jun
21

# Eventss

2017
Sep
12

# T&G: Liat Kessler (Cornell and Oranim), Extending Homologically trivial symplectic cyclic actions to Hamiltonian circle actions

12:00pm to 1:00pm

## Location:

Ross Building Room 70A

We ask whether every homologically trivial cyclic action on a symplectic four-manifold extend to a Hamiltonian circle action. By a cyclic action we mean an action of a cyclic group of finite order; it is homologically trivial if it induces the identity map on homology. We assume that the manifold is closed and connected. In the talk, I will give an example of a homologically trivial symplectic cyclic action on a four-manifold that admits Hamiltonian circle actions, and show that is does not extend to a Hamiltonian circle action.

2016
Jun
15

# Topology & geometry, Ezra Getzler (Northwestern University), "The derived Maurer-Cartan locus"

2:00pm to 3:35pm

## Location:

Ross building, Hebrew University (Seminar Room 70A)

Abstract:

Derived algebraic geometry is a nonlinear analogue of homological algebra, in which one keeps track of syzygies among the relations among the defining equations of a variety, and higher analogues. It has important applications to intersection theory and enumerative geometry.

Derived algebraic geometry is a nonlinear analogue of homological algebra, in which one keeps track of syzygies among the relations among the defining equations of a variety, and higher analogues. It has important applications to intersection theory and enumerative geometry.

2017
May
23

# Topology & Geometry Seminar: Adina Gamse (University of Toronto), "Vanishing relations in the cohomology of the moduli space of parabolic bundles".

1:00pm to 1:50pm

## Location:

Ross A70.

Abstract: Let \Sigma be a compact connected oriented 2-manifold of genus g , and let p be a point on \Sigma. We define a space S_g(t) consisting of certain irreducible representations of the fundamental group of \Sigma - { p } , modulo conjugation by SU(N).

2017
Aug
09

# T&G: Peter Ozsvath (Princeton), Bordered methods in knot Floer homology

12:00pm to 1:00pm

## Location:

Room 70A, Ross Building, Jerusalem, Israel

Knot Floer homology is an invariant for knots in the three-sphere defined using methods from symplectic geometry. I will describe a new algebraic formulation of this invariant which leads to a reasonably efficient computation of these invariants. This is joint work with Zoltan Szabo.

2015
Nov
05

# Groups & Dynamics : Ilya Khayutin (HUJI)

9:45am to 11:00am

## Location:

Manchester building, Hebrew University of Jerusalem, (Room 209)

Title: Arithmetic of Double Torus Quotients and the Distribution of Periodic Torus Orbits
Abstract:
In this talk I will describe some new arithmetic invariants for pairs of torus orbits on inner forms of PGLn and SLn. These invariants allow us to significantly strengthen results towards the equidistribution of packets of periodic torus orbits on higher rank S-arithmetic quotients. An important aspect of our method is that it applies to packets of periodic orbits of maximal tori which are only partially split.

2016
Apr
10

# Dvoretzky lecture 2: Lai-Sang Young (Courant) "Proving the positivity of Lyapunov exponents"

4:00pm to 5:00pm

## Location:

Lecture hall 2

A signature of chaotic behavior in dynamical systems is sensitive dependence on initial conditions, and Lyapunov exponents measure the rates at which nearby orbits diverge. One might expect that geometric expansion or stretching in a map would lead to positive Lyapunov exponents. This, however, is very difficult to prove - except for maps with invariant cones (or a priori separation of expanding and contracting directions).

2015
Dec
17

# Groups & dynamics: Robert Hough (IAS) - Mixing and cut-off on cyclic groups

12:00pm to 1:00pm

## Location:

Einstein 110

Consider a sequence of random walks on $\mathbb{Z}/p\mathbb{Z}$ with symmetric generating sets $A= A(p)$. I will describe known and new results regarding the mixing time and cut-off. For instance, if the sequence $|A(p)|$ is bounded then the cut-off phenomenon does not occur, and more precisely I give a lower bound on the size of the cut-off window in terms of $|A(p)|$. A natural conjecture from random walk on a graph is that the total variation mixing time is bounded by maximum degree times diameter squared.

2015
Nov
19

# Groups & dynamics: Lei Yang (HUJI) "Equidistribution of expanding translates of curves in homogeneous spaces and Diophantine approximation"

10:00am to 11:00am

## Location:

Ross 70

Title: Equidistribution of expanding translates of curves in homogeneous spaces and Diophantine approximation.
Abstract:
We consider an analytic curve $\varphi: I \rightarrow \mathbb{M}(n\times m, \mathbb{R}) \hookrightarrow \mathrm{SL}(n+m, \mathbb{R})$ and embed it into some homogeneous space $G/\Gamma$, and translate it via some diagonal flow

2015
Nov
12

# Groups & dynamics: Elon Lindenstrauss (HUJI), "Rigidity of higher rank diagonalizable actions in positive characteristic"

10:00am to 11:00am

## Location:

Ross 70

Title: Rigidity of higher rank diagonalizable actions in positive characteristic

2015
Nov
03

# Dynamics & probability: Asaf Nachmias (Tel Aviv)

2:00pm to 3:00pm

## Location:

Manchester building, Hebrew University of Jerusalem, (Room 209)

Title: Indistinguishability of trees in uniform spanning forests

Abstract:

The uniform spanning forest (USF) of an infinite connected graph G is the weak limit of the uniform spanning tree measure taken on exhausting finite subgraphs of G. It is easy to see that it is supported on spanning graphs of G with no cycles, but it need not be connected. Indeed, a classical result of Pemantle ('91) asserts that when G=Zd, the USF is almost surely a connected tree if and only if d=1,2,3,4.

Abstract:

The uniform spanning forest (USF) of an infinite connected graph G is the weak limit of the uniform spanning tree measure taken on exhausting finite subgraphs of G. It is easy to see that it is supported on spanning graphs of G with no cycles, but it need not be connected. Indeed, a classical result of Pemantle ('91) asserts that when G=Zd, the USF is almost surely a connected tree if and only if d=1,2,3,4.

2015
Dec
29

2015
Dec
08

# Dynamics & probability: Brandon Seward (HUJI): "Positive entropy actions of countable groups factor onto Bernoulli shifts"

2:00pm to 3:00pm

## Location:

Manchester building, Hebrew University of Jerusalem, (Room 209)

Title: Positive entropy actions of countable groups factor onto Bernoulli shifts

Abstract: I will prove that if a free ergodic action of a countable group has positive Rokhlin entropy (or, less generally, positive sofic entropy) then it factors onto all Bernoulli shifts of lesser or equal entropy. This extends to all countable groups the well-known Sinai factor theorem from classical entropy theory. As an application, I will show that for a large

class of non-amenable groups, every positive entropy free ergodic action satisfies the measurable von Neumann conjecture.

Abstract: I will prove that if a free ergodic action of a countable group has positive Rokhlin entropy (or, less generally, positive sofic entropy) then it factors onto all Bernoulli shifts of lesser or equal entropy. This extends to all countable groups the well-known Sinai factor theorem from classical entropy theory. As an application, I will show that for a large

class of non-amenable groups, every positive entropy free ergodic action satisfies the measurable von Neumann conjecture.

2015
Dec
22

# Dynamics & probability: Naomi Feldheim (Stanford), "New results on zeroes of stationary Gaussian functions"

2:00pm to 3:00pm

## Location:

Math 209 (Manchester building)

We consider (complex) Gaussian analytic functions on a horizontal strip, whose distribution is invariant with respect to horizontal shifts (i.e., "stationary"). Let N(T) be the number of zeroes in [0,T] x [a,b]. We present an extension of a result by Wiener, concerning the existence and characterization of the limit N(T)/T as T approaches infinity, as well as characterize the growth of the variance of N(T).

2017
Feb
28

# Dynamics seminar: Emmanuel Roy (Paris 13): Ergodic splittings of Poisson processes

2:00pm to 3:00pm

If N denotes a Poisson process, a splitting of N is formed by two point processes N_1 and N_2 such that N=N_1+N_2.

If N_1 and N_2 are independent Poisson processes then the splitting is said to be Poisson and such a splitting is always available (We allow the possibility to enlarge the ambient probability space).

In general, a splitting is not Poisson but the situation changes if we require that the distributions of the point processes involved are left invariant by a common underlying map that acts at the level of each point of the processes.

If N_1 and N_2 are independent Poisson processes then the splitting is said to be Poisson and such a splitting is always available (We allow the possibility to enlarge the ambient probability space).

In general, a splitting is not Poisson but the situation changes if we require that the distributions of the point processes involved are left invariant by a common underlying map that acts at the level of each point of the processes.