2016
Mar
22

# Events & Seminars

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.

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
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
Jun
08

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:

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:

Abstract:

2016
Mar
23

# Topology & geometry, Amitai Zernik (Hebrew University), "Fixed-point Expressions for Open Gromov-Witten Invariants - overview and $A_{\infty}$ perspective"

11:00am to 12:45pm

## Location:

Ross building, Hebrew University (Seminar Room 70A)

Abstract:

In this pair of talks I will discuss how to obtain fixed-point expressions

for open Gromov-Witten invariants. The talks will be self-contained,

and the second talk will only require a small part of the first talk,

which we will review.

The Atiyah-Bott localization formula has become a valuable tool for

computation of symplectic invariants given in terms of integrals on

the moduli spaces of closed stable maps. In contrast, the moduli spaces

of open stable maps have boundary which must be taken into account

In this pair of talks I will discuss how to obtain fixed-point expressions

for open Gromov-Witten invariants. The talks will be self-contained,

and the second talk will only require a small part of the first talk,

which we will review.

The Atiyah-Bott localization formula has become a valuable tool for

computation of symplectic invariants given in terms of integrals on

the moduli spaces of closed stable maps. In contrast, the moduli spaces

of open stable maps have boundary which must be taken into account

2016
Jan
20

# Topology & geometry, Matan Prasma (Radboud University), "Model-categorical cotangent complex formalism"

11:00am to 12:45pm

## Location:

Ross building, Hebrew University (Seminar Room 70A)

Abstract: One of the first applications of model categories was Quillen homology. Building on the notion of Beck modules, one defines the cotangent complex of an associative or commutative (dg)-algebras as the derived functor of its abelianization. The latter is a module over the original algebra, and its homology groups are called the (Andre'-)Quillen homology. The caveat of this approach is that the cotangent complex is not defined as a functor on the category of all algebras.

2015
Nov
11

# Topology & geometry: Cy Maor (HUJI), "Limits of elastic energies of converging Riemannian manifolds"

11:00am to 12:45pm

## Location:

Ross building, Hebrew University (Seminar Room 70A)

Abstract: An elastic energy functional of a Riemannian manifold is a function that measures the distance of an embedding u:→ℝd from being isometric. In many applications, the manifold in consideration is actually a limit of other manifolds, that is, is a limit of n in some sense. Assuming that we have an elastic energy functional for each n, can we obtain an energy functional of which is a limit of the functionals of n?

2015
Dec
16

# Topology & geometry: Yochay Jerby (HUJI), " Exceptional collections on toric Fano manifolds and the Landau-Ginzburg equations"

11:00am to 2:30pm

## Location:

Ross building, Hebrew University (Seminar Room 70A)

Abstract: For a toric Fano manifold $X$ denote by $Crit(X) \subset (\mathbb{C}^{\ast})^n$ the solution scheme of the Landau-Ginzburg system of equations of $X$. Examples of toric Fano manifolds with $rk(Pic(X)) \leq 3$ which admit full strongly exceptional collections of line bundles were recently found by various authors. For these examples we construct a map $E : Crit(X) \rightarrow Pic(X)$ whose image $\mathcal{E}=\left \{ E(z) \vert z \in Crit(X) \right \}$ is a full strongly exceptional collection satisfying the M-aligned property.

2016
Mar
09

# Topology & geometry, Frol Zapolsky (University of Haifa), "On the contact mapping class group of the prequantization space over the Am Milnor fiber"

11:00am to 12:45pm

## Location:

Ross building, Hebrew University (Seminar Room 70A)

Abstract: The contact mapping class group of a contact manifold V is the set of contact isotopy classes of its contactomorphisms. When V is the 2n-dimensional (n at least 2) Am Milnor fiber times the circle, with a natural contact structure, we show that the full braid group Bm+1 on m+1strands embeds into the contact mapping class group of V. We deduce that when n=2, the subgroup Pm+1 of pure braids is mapped to the part of the contact mapping class group consisting of smoothly trivial classes. This solves the contact isotopy problem for V.

2016
Jun
15

# Topology & geometry, Vasily Dolgushev (Temple University), "The Intricate Maze of Graph Complexes"

11:00am to 12:45pm

## Location:

Ross building, Hebrew University (Seminar Room 70A)

Abstract: In the paper "Formal noncommutative symplectic geometry'', Maxim Kontsevich introduced three versions of cochain complexes GCCom, GCLie and GCAs "assembled from'' graphs with some additional structures. The graph complex GCCom (resp. GCLie, GCAs) is related to the operad Com (resp. Lie, As) governing commutative (resp. Lie, associative) algebras. Although the graphs complexes GCCom, GCLie and GCAs (and their generalizations) are easy to define, it is hard to get very much information about their cohomology spaces.

2016
Mar
02

# Topology & geometry, Dmitry Tonkonog (University of Cambridge), "Monotone Lagrangian tori and cluster mutations"

11:00am to 12:45pm

## Location:

Ross building, Hebrew University (Seminar Room 70A)

Abstract: I will review a beautiful construction of an infinite collection of monotone Lagrangian tori in the projective plane (and other del Pezzo surfaces) due to Renato Vianna. These tori are obtained from a single one by a procedure called mutation, and I will talk about the wall-crossing formula which relates this geometric procedure to algebraic mutation known from cluster algebra. A proof of the wall-crossing formula is work in progress.