Eventss

2016 Jan 05

Dynamics lunch: Sebastian Donoso (HUJI) - Automorphism groups of low complexity subshifts

12:00pm to 1:00pm

Location: 

Manchester building, Hebrew University of Jerusalem, (Coffee lounge)
Abstract: The automorphism group of a subshift $(X,\sigma)$ is the group of homeomorphisms of $X$ that commute with $\sigma$. It is known that such groups can be extremely large for positive entropy subshifts (like full shifts or mixing SFT). In this talk I will present some recent progress in the understanding of the opposite case, the low complexity one. I will show that automorphism groups are highly constrained for low complexity subshifts. For instance, for a minimal subshifts with sublinear complexity the automorphism group is generated by the shift and a finite set.
2016 Jan 12

Dynamics lunch: Brandon Seward (HUJI), "Borel chromatic numbers of free groups"

12:00pm to 1:00pm

Location: 

Manchester building, Hebrew University of Jerusalem, (Coffee lounge)
Borel chromatic numbers of free groups Abstract: Recall that a coloring of a graph is a labeling of its vertices such that no pair of vertices joined by an edge have the same label. The chromatic number of a graph is the smallest number of colors for which there is a coloring. If G is a finitely generated group with generating set S, then for any free action of G on a standard Borel space X, we can place a copy of the S-Cayley graph of G onto every orbit. This results in a graph whose vertex set is X and whose edge set is Borel measurable. We can then consider Borel
2016 Mar 08

Dynamics lunch seminar: Brandon Seward (HUJI): Entropy theory for non-amenable groups (part I)

12:00pm to 1:45pm

Location: 

Ross 70
Entropy was first defined for actions of the integers by Kolmogorov in 1958 and then extended to actions of countable amenable groups by Kieffer in 1975. Recently, there has been a surge of research in entropy theory following groundbreaking work of Lewis Bowen in 2008 which defined entropy for actions of sofic groups. In this mini-course I will cover these recent developments. I will carefully define the notions of sofic entropy (for actions of sofic groups) and Rokhlin entropy (for actions of general countable groups), discuss many of the main results, and go through some of the proofs.
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.
2016 May 25

Topology & geometry, Richard Bamler (UC Berkeley), "There are finitely many surgeries in Perelman's Ricci flow"

11:00am to 12:45pm

Location: 

Ross building, Hebrew University (Seminar Room 70A)
Abstract: Although the Ricci flow with surgery has been used by Perelman to solve the Poincaré and Geometrization Conjectures, some of its basic properties are still unknown. For example it has been an open question whether the surgeries eventually stop to occur (i.e. whether there are finitely many surgeries) and whether the full geometric decomposition of the underlying manifold is exhibited by the flow as t→∞.
2016 Mar 30

Topology & geometry, Amitai Zernik (Hebrew University), "Fixed-point Expressions for Open Gromov-Witten Invariants - idea of the proof"

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 order to apply fixed-point localization. Homological perturbation

Pages