Events & Seminars

2017 Mar 23

Colloquium: Asaf Shapira (Tel Aviv) - "Removal Lemmas with Polynomial Bounds"

2:30pm to 3:30pm

Location: 

Manchester Building (Hall 2), Hebrew University Jerusalem
A common theme in many extremal problems in graph theory is the relation between local and global properties of graphs. We will consider the following variant of this theme: suppose a graph G is far (in some well defined sense) from satisfying property P. Must G contain a small proof of this fact? We will show that for many natural graph properties the answer is Yes. In particular, we will show that the answer is Yes whenever P is a semi-algebraic graph property, thus conforming a conjecture of Alon. Joint work with L. Gishboliner
2015 Nov 25

Topology & geometry: Lara Simone Suárez (HUJI), "Exact Lagrangian cobordism and pseudo-isotopy"

11:00am to 12:45pm

Location: 

Ross building, Hebrew University (Seminar Room 70A)
Abstract: Consider two Lagrangian submanifolds L, L′ in a symplectic manifold (M,ω). A Lagrangian cobordism (W;L,L′) is a smooth cobordism between L and L′ admitting a Lagrangian embedding in (([0,1]×R)×M,(dx∧dy)⊕ω) that looks like [0,ϵ)×{1}×L and (1−ϵ,1]×{1}×L′ near the boundary. In this talk we will show that under some topological constrains, an exact Lagrangian cobordism (W;L,L′) with dim(W)>5 is diffeomorphic to [0,1]×L.
2017 Jun 01

Group actions:Lei Yang - badly approximable points on curves and unipotent orbits in homogeneous spaces

10:30am to 11:30am

We will study n-dimensional badly approximable points on curves. Given an analytic non-degenerate curve in R^n, we will show that any countable intersection of the sets of weighted badly approximable points on the curve has full Hausdorff dimension. This strengthens a previous result of Beresnevich by removing the condition on weights. Compared with the work of Beresnevich, we study the problem through homogeneous dynamics. It turns out that the problem is closely related to the study of distribution of long pieces of unipotent orbits in homogeneous spaces.
2017 Apr 20

Basic notions: Raz Kupferman (HUJI) - A geometric framework for continuum mechanics

4:00pm to 5:15pm

Abstract: The “geometrization" of mechanics (whether classical, relativistic or quantum) is almost as old as modern differential geometry, and it nowadays textbook material. The formulation of a mathematically-sound theory for the mechanics of continuum media is still a subject of ongoing research. In this lecture I will present a geometric formulation of continuum mechanics, starting with the definition of the fundamental physical observables, e.g., force, deformation, stress and traction. The outcome of this formulation is a generalization of Newton’s "F=ma” equation for continuous media.
2016 Dec 08

Colloquium: Gordon Slade (UBC) "Critical phenomena in statistical mechanics"

2:30pm to 3:30pm

Location: 

Manchester Building (Hall 2), Hebrew University Jerusalem
The subject of phase transitions and critical phenomena in statistical mechanics is a rich source of interesting and difficult mathematical problems. There has been considerable success in solving such problems for systems in spatial dimension 2, or in high dimensions, but not in dimension 3. This lecture is intended to provide an introduction to recent work that employs a renormalisation group method to study spin systems and self-avoiding walk in dimension 4 (joint with Bauerschmidt and Brydges), as well as long-range versions of these models in dimensions 1,2,3 via an "epsilon expansion."
2015 Dec 31

Colloquium : Jasmin Matz, "Distribution of Hecke eigenvalues for GL(n)"

2:30pm to 3:30pm

Location: 

Manchester Building (Hall 2), Hebrew University Jerusalem
Abstract: A classical problem in the theory of automorphic forms is to count the number of eigenvalues of the Laplacian acting on cuspidal functions on the quotient of the upper half plane by a lattice R. For R=SL(2,Z) (or a congruence subgroup thereof) the answer is given by Selberg's Weyl law while for the higher rank situation it was established by Mueller and Lindenstrauss-Venkatesh. Additional to the Laplacian there is another large family of operators, namely the Hecke operators attached to SL(2,Z). Sarnak proved that the
2016 Apr 07

Colloquium: Lai-Sang Young (Courant) "Measuring Dynamical Complexity"

2:30pm to 3:30pm

Location: 

Manchester Building (Hall 2), Hebrew University Jerusalem
I will discuss, for differentiable dynamical systems, three ways to capture dynamical complexity: hyperbolicity, which measures the sensitivity of dependence on initial conditions. entropy, which measures the predictability of future dynamical events in the sense of information theory. the speed of correlation decay or equivalently the rate at which memory is lost.
2016 Nov 24

Colloquium: Dan Freed (University of Texas) "Bordism and topological phases of matter"

2:00pm to 3:00pm

Location: 

Manchester Building (Hall 2), Hebrew University Jerusalem
Topological ideas have at various times played an important role in condensed matter physics. This year's Nobel Prize recognized the origins of a particular application of great current interest: the classification of phases of a quantum mechanical system. Mathematically, we would like describe them as path components of a moduli space, but that is not rigorously defined as of now. In joint work with Mike Hopkins we apply stable homotopy theory (Adams spectral sequence) to compute the group of topological phases of "invertible" systems. We posit a continuum field
2016 Mar 17

Colloquium-Landau Lectures: Ravi Vakil (Stanford) "Cutting and pasting in algebraic geometry, and the motivic zeta function"

3:30pm to 4:30pm

Location: 

Manchester Building (Hall 2), Hebrew University Jerusalem
Given some class of "geometric spaces", we can make a ring as follows. 1. (additive structure) When U is an open subset of such a space X, [X] = [U] + [(X \ U)] 2. (multiplicative structure)} [X x Y] = [X] [Y]. In the algebraic setting, this ring (the "Grothendieck ring of varieties") contains surprising structure, connecting geometry to arithmetic and topology.
2015 Nov 26

Colloquium: Shai Evra (HUJI), "Topological Expanders"

2:30pm to 3:30pm

Location: 

Manchester Building (Hall 2), Hebrew University Jerusalem
Title: Topological Expanders. Abstract: A classical result of Boros-Furedi (for d=2) and Barany (for d>=2) from the 80's, asserts that given any n points in R^d, there exists a point in R^d which is covered by a constant fraction (independent of n) of all the geometric (=affine) d-simplices defined by the n points. In 2010, Gromov strengthen this result, by allowing to take topological d-simplices as well, i.e. drawing continuous lines between the n points, rather then straight lines and similarly continuous simplices rather than affine.

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