Eventss

2017 Jun 29

Special Seminar: Ayala Byron (HUJI) "Homogeneity of torsion-free hyperbolic groups"

2:00pm to 3:00pm

Location: 

Ross 70

Abstract:
A (countable) group G is homogeneous if whenever g,h are tupples of the same type in G, there is an automorphism of G sending g to h.
We give a characterization of freely-indecomposable torsion-free hyperbolic groups which are homogeneous, in terms of a particular decomposition as a graph of groups - their JSJ decomposition. This is joint work with Chloe Perin.
2017 Jun 13

Topology and Geometry Seminar: Alexander Caviedes Castro (Tel-Aviv University), "Symplectic capacities and Cayley graphs"

1:00pm to 1:50pm

Location: 

Ross 70A
Abstract: The Gromov non-squeezing theorem in symplectic geometry states that is not possible to embed symplectically a ball into a cylinder of smaller radius, although this can be done with a volume preserving embedding. Hence, the biggest radius of a ball that can be symplectically embedded into a symplectic manifold can be used as a way to measure the "symplectic size'' of the manifold. We call the square of this radius times the number \pi the Gromov width of the symplectic manifold. The Gromov width as a symplectic invariant is extended through the notion of "Symplectic Capacity".
2017 May 10

Peli Grietzer (Harvard literature dept.)

4:00pm to 5:00pm

Location: 

Ross 70.
Abstract:
In 1962, amateur literary theorist and professional mathematician Andrey Kolmogorov wrote: ‘A story is art only if the characters and situations it describes stand a chance of becoming
2017 Dec 18

HD-Combinatorics: Steven Damelin, "Approximate and exact alignment of data, extensions and interpolation in R^D--parts"

2:00pm to 4:00pm

Location: 

Sprinzak Building, Room 28
Speaker: Steven Damelin (The American Mathematical Society)
Abstract:
A classical problem in geometry goes as follows. Suppose we are given two sets of $D$ dimensional data, that is, sets of points in $R^D$. The data sets are indexed
by the same set, and we know that pairwise distances between corresponding points are equal in the two data sets. In other words, the sets are isometric. Can this correspondence be extended
to an isometry of the ambient Euclidean space?
In this form the question is not terribly interesting; the answer has long known
2017 Dec 25

HD-Combinatorics: Shai Evra, "Bounded degree high dimensional expanders"

2:00pm to 4:00pm


In the recent theory of high dimensional expanders, the following open problem was raised by Gromov: Are there bounded degree high dimensional expanders?
For the definition of high dimensional expanders, we shall follow the pioneers of this field, and consider the notions of coboundary expanders (Linial-Meshulam) and topological expanders (Gromov).
In a recent work, building on an earlier work of Kaufman-Kazhdan-Lubotzky in dimension 2, we were able to prove the existence of bounded degree expanders according to Gromov, in every dimension.
2017 Nov 20

Leonard Schulman, "Analysis of a Classical Matrix Preconditioning Algorithm"

2:00pm to 3:00pm

Location: 

Room 130, Feldman Building, Givat Ram
There are several prominent computational problems for which
simple iterative methods are widely preferred in practice despite
an absence of runtime or performance analysis (or "worse", actual
evidence that more sophisticated methods have superior
performance according to the usual criteria). These situations
raise interesting challenges for the analysis of algorithms.
We are concerned in this work with one such simple method: a
classical iterative algorithm for balancing matrices via scaling
2017 Oct 23

HD-Combinatorics: Nati Linial, "High-dimensional permutations"

2:00pm to 4:00pm

Location: 

Israel Institute for Advanced Studies (Feldman building, Givat Ram), Eilat Hall
This is a survey talk about one of the main parts of what we call high-dimensional combinatorics. We start by equating a permutation with a permutation matrix. Namely, an nxn array of zeros and ones where every line (=row or column) contains exactly one 1. In general, a d-dimensional permutation is an array [n]x[n]x....x[n] (d+1 factors) of zeros and ones in which every line (now there are d+1 types of lines) contains exactly one 1. Many questions suggest themselves, some of which we have already solved, but many others are still wide opne. Here are a few examples:
2017 Nov 13

HD-Combinatorics: Shmuel Weinberger, "L^2 cohomology"

2:00pm to 4:00pm

Location: 

Room 130, Feldman Building, Givat Ram
Abstract: I will give an introduction to the cohomology of universal covers of finite complexes. These groups are (for infinite covers) either trivial or infinite dimensional, but they have renormalized real valued Betti numbers. Their study is philosophically related to the topic of our year, and they have wonderful applications in geometry, group theory, topology etc and I hope to explain some of this.

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