2015 Dec 30

# Topology & geometry, Amitai Yuval (HUJI), " Geodesics of symmetric positive Lagrangians"

11:00am to 12:45pm

## Location:

Ross building, Hebrew University (Seminar Room 70A)
Abstract: A Hamiltonian isotopy class of positive Lagrangians in an almost Calabi-Yau manifold admits a natural Riemannian metric. This metric has a Levi-Civita connection, and hence, it gives rise to a notion of geodesics. The geodesic equation is fully non-linear degenerate elliptic, and in general, it is yet unknown whether the initial value problem and boundary problem are well-posed. However, results on the existence of geodesics could shed new light on special Lagrangians, mirror symmetry and the strong Arnold conjecture.
2015 Nov 18

# Topology & geometry: Lara Simone Suárez (HUJI), "Whitehead torsion and s-cobordism theorem"

11:00am to 12:45pm

## Location:

Ross building, Hebrew University (Seminar Room 70A)
Abstract: We will give a beginner's introduction to simple homotopy theory and explain how it applies to prove the s-cobordism theorem, a generalization of the h-cobordism theorem for non-simply-connected h-cobordisms.
2016 Mar 16

# Topology & geometry, Sara Tukachinsky (Hebrew University), "Point-like bounding chains in open Gromov-Witten theory"

11:00am to 12:45pm

## Location:

Ross building, Hebrew University (Seminar Room 70A)
Abstract:
Over a decade ago Welschinger defined invariants of real symplectic manifolds of complex dimensions 2 and 3, which count $J$-holomorphic disks with boundary and interior point constraints. Since then, the problem of extending the definition to higher dimensions has attracted much attention.
2015 Dec 23

# Topology & geometry: Oren Ben-Bassat (Oxford University), "Multiple Lagrangian Intersections"

11:00am to 12:45pm

## Location:

Ross building, Hebrew University (Seminar Room 70A)
Abstract: Joyce and others have used shifted symplectic geometry to define Donaldson-Thomas Invariants. This kind of geometry naturally appears on derived moduli stacks of perfect complexes on Calabi-Yau varieties. One wonderful feature of shifted symplectic geometry (developed by Pantev, Toën, Vaquié and Vezzosi) is that fibre products (i.e. intersections) of Lagrangians automatically carry Lagrangian structures. Using a strange property of triple intersections from arXiv:1309.0596, this extra structure can be organized into a 2-category.
2016 Jan 13

# Topology & geometry, Penka Vasileva (Paris Rive Gauche), "Real Gromov-Witten theory in all genera"

11:00am to 12:45pm

## Location:

Ross building, Hebrew University (Seminar Room 70A)
Abstract: We construct positive-genus analogues of Welschinger's invariants for many real symplectic manifolds, including the odd-dimensional projective spaces and the quintic threefold. Our approach to the orientability problem is based entirely on the topology of real bundle pairs over symmetric surfaces. This allows us to endow the uncompactified moduli spaces of real maps from symmetric surfaces of all topological types with natural orientations and to verify that they extend across the codimension-one boundaries of these spaces.
2015 Nov 04

# Topology & geometry: Chaim Even Zohar (HUJI), "Invariants of Random Knots"

11:00am to 12:45pm

## Location:

Ross building, Hebrew University (Seminar Room 70A)
Title: Invariants of Random Knots.
Abstract:
Random curves in space and how they are knotted give an insight into the behavior of "typical" knots and links, and are expected to introduce the probabilistic method into the mathematical study of knots. They have been studied by biologists and physicists in the context of the structure of random polymers. There have been many results obtained via computational experiment, but few explicit computations.
2016 Jun 08

# Topology & geometry, Ailsa Keating (Columbia University), "Homological Mirror Symmetry for singularities of type Tpqr"

11:00am to 12:45pm

## Location:

Ross building, Hebrew University (Seminar Room 70A)
Abstract:
2016 Jan 06

# Topology & geometry, Egor Shelukhin (IAS), "The L^p diameter of the group of area-preserving diffeomorphisms of S^2"

11:00am to 12:45pm

## Location:

Ross building, Hebrew University (Seminar Room 70A)
Abstract: We use a geometric idea to give an analytic estimate for the word-length in the pure braid group of S^2. This yields that the L^1-norm (and hence each L^p-norm, including L^2) on the group of area-preserving diffeomorphisms of S^2 is unbounded. This solves an open question arising from the work of Shnirelman and Eliashberg-Ratiu. Joint work in progress with Michael Brandenbursky.
2016 Feb 24

# Topology & geometry, Mikhail Katz (Bar Ilan University), "Determinantal variety and bi-Lipschitz equivalence"

11:00am to 12:45pm

## Location:

Ross building, Hebrew University (Seminar Room 70A)
Abstract: The unit circle viewed as a Riemannian manifold has diameter (not 2 but rather) π, illustrating the difference between intrinsic and ambient distance. Gromov proceeded to erase the difference by pointing out that when a Riemannian manifold is embedded in L∞, the intrinsic and the ambient distances coincide in a way that is as counterintuitive as it is fruitful. Witness the results of his 1983 Filling paper.
2015 Dec 02

# Topology & geometry: Pavel Paták (HUJI), "Homological non-embeddability and a qualitative topological Helly-type theorem"

11:00am to 12:45pm

## Location:

Ross building, Hebrew University (Seminar Room 70A)
Abstract: The classical theorem of Van Kampen and Flores states that the k-dimensional skeleton of (2k+2)-dimensional simplex cannot be embedded into R2k.
We present a version of this theorem for chain maps and as an application we prove a qualitative topological Helly-type theorem.
If we define the Helly number of a finite family of sets to be one if all sets in the family have a point in common and as the largest size of inclusion-minimal subfamily with empty intersection otherwise, the theorem can be stated as follows:
2016 Apr 06

# Topology & geometry, Sari Ghanem (Université Joseph Fourier - Grenoble I ), "The decay of SU(2) Yang-Mills fields on the Schwarzschild black hole with spherically symmetric small energy initial data"

11:00am to 12:45pm

## Location:

Levi building, Hebrew University ( Room 06)
**Note the special location**
Abstract:
2016 Feb 17

# Menachem Magidor 70th Birthday Conference

Wed, 17/02/2016 (All day) to Fri, 19/02/2016 (All day)

2016 Dec 15

# Groups and dynamics: Yair Hartman (Northwestern) - Percolation, Invariant Random Subgroups and Furstenberg Entropy

10:30am to 11:30am

## Location:

Ross 70
Abstract:
In this talk I'll present a joint work with Ariel Yadin, in which we solve the Furstenberg Entropy Realization Problem for finitely supported random walks (finite range jumps) on free groups and lamplighter groups. This generalizes a previous result of Bowen. The proof consists of several reductions which have geometric and probabilistic flavors of independent interests.
All notions will be explained in the talk, no prior knowledge of Invariant Random Subgroups or Furstenberg Entropy is assumed.
2015 Dec 31

# Groups & dynamics: Thang Neguyen (Weizmann) - Rigidity of quasi-isometric embeddings

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 31

# Groups & dynamics: Paul Nelson (ETH) - Quantum variance on quaternion algebras

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