Events & Seminars

2017 May 24

Mark Rudelson: Delocalization of the eigenvectors of random matrices.

2:00pm to 3:00pm

Location: 

רוס 63
Abstract: Consider a random matrix with i.i.d. normal entries. Since its distribution is invariant under rotations, any normalized eigenvector is uniformly distributed over the unit sphere. For a general distribution of the entries, this is no longer true. Yet, if the size of the matrix is large, the eigenvectors are distributed approximately uniformly. This property, called delocalization, can be quantified in various senses. In these lectures, we will discuss recent results on delocalization for general random matrices.
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 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. ‏האירוע הזה כולל שיחת וידאו ב-Google Hangouts.
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".
2018 Jan 08

HD-Combinatorics: Amnon Ta-Shma, "Bias samplers and reducing overlap in random walks over graphs"

2:00pm to 4:00pm

Abstract: 

The expander Chernoff bound states that random walks over expanders are good samplers, at least for a certain range of parameters. In this talk we will be interested in “Parity Samplers” that have the property that for any test set, about half of the sample sets see the test set an *even* number of times, and we will check whether random walks over expanders are good parity samplers. We will see that:

1. Random walks over expanders fare quite well with the challenge, but,
2. A sparse Random complex does much better.
2017 Nov 27

HD-Combinatorics: Irit Dinur, "PCPs and high dimensional expansion"

2:00pm to 4:00pm

Location: 

Room 130, Feldman Building (IIAS), Givat Ram
The "PCP theorem" says that problems in NP are hard in a robust or stable way. I will give a brief intro to PCPs (and explain the acronym) and then try to outline a proof of the PCP theorem based on "agreement expansion" which is a form of high dimensional expansion. My aim is to show how high dimensional expansion is inherently present in PCP type questions.
2017 Sep 05

IIAS Seminar: Tatiana Nagnibeda - Infinite Ramanujan graphs and completely dissipative actions

4:00pm to 5:00pm

Location: 

Math room 209
Speaker : Tatiana Nagnibeda (University of Geneva) Abstract: The definition of a Ramanujan graph extends naturally to infinite graphs: an infinite graph is Ramanujan if its spectral radius is not larger than (and hence equal to) the spectral radius of its universal covering tree. As with infinite families of finite graphs, it is interesting and non-trivial to understand, how much Ramanujan graphs resemble trees. I will discuss some results in this direction obtained in a joint work with Vadim Kaimanovich, by investigating ergodic properties of boundary actions of free groups.
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

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