2017 Nov 02

Colloquium: Michael Brandenbursky (BGU) "Entropy, metrics and quasi-morphisms."

2:30pm to 3:30pm


Manchester Building (Hall 2), Hebrew University Jerusalem
One of the mainstream and modern tools in the study of non abelian groups are quasi-morphisms. These are functions from a group to the reals which satisfy homomorphism condition up to a bounded error. Nowadays they are used in many fields of mathematics. For instance, they are related to bounded cohomology, stable commutator length, metrics on diffeomorphism groups, displacement of sets in symplectic topology, dynamics, knot theory, orderability, and the study of mapping class groups and of concordance group of knots.
2017 Dec 18

SPECIAL Jerusalem Analysis and PDEs seminar: "Steady Water Waves" Walter Strauss (Brown University)

4:00pm to 5:00pm


Ross 63
I will consider classical 2D traveling water waves with vorticity. By
means of local and global bifurcation theory using topological degree,
we can prove that there exist many such waves. They are exact smooth
solutions of the Euler equations with the physical boundary conditions.
Some of the waves are quite tall and steep and some are overhanging.
There are periodic ones and solitary ones. I will exhibit some
numerical computations of such waves. New analytical results will be
presented on waves with favorable vorticity.
2015 Oct 22

Colloquium: Nir Avni (Northwestern), "Counting points and counting representations"

2:30pm to 3:30pm

Title: Counting points and counting representations
I will talk about the following questions:
1) Given a system of polynomial equations with integer coefficients, how many solutions does it have in the ring Z/N?
2)​ Given a polynomial map f:R^a-->R^b and a smooth, compactly supported measure m on R^a, does the push-forward of m by f have bounded density?
3) Given a lattice in a higher rank Lie group (say, SL(n,Z) for n>2). How many d-dimensional representations does it ​have?
2017 Jan 22

Special colloquium: Laci Babai (Chicago) "Graph isomorphism and coherent configurations: The Split-or-Johnson routine"

4:00pm to 6:00pm


Rothberg B220 (CS bldg)
Coherent configurations" (CCs) are certain highly regular colorings of the directed complete graph. The concept goes back to Schur (1933) who used it to study permutation groups, and has subsequently been rediscovered in other contexts (block designs,
association schemes, graph canonization).
CCs are the central concept in the "Split-or-Johnson" (SoJ) procedure, one of the main combinatorial components of the speaker's recent algorithm to test graph isomorphism.
2015 Oct 29

Colloquium: Vincent Guirardel (Universite de Renne 1), "Avatars of small cancellation"

2:30pm to 3:30pm

Title: Avatars of small cancellation
In general, given a finite presentation of a group, it is very difficult (in fact algorithmically impossible) to understand the group it defines. Small cancellation theory was developped as a combinatorial condition on a presentation that allows one to understand the group it represents. This very flexible construction has many applications to construct examples of groups with specific features.