2018
Feb
05

# Eventss

2018
Feb
26

# HD-Combinatorics Special Day on On Random groups and property T (organized by Izhar Oppenheim)

(All day)

## Location:

Room 130, IIAS, Feldman Building, Givat Ram

10:00-11:00:

**Izhar Oppenheim**, Introduction to Property (T)

11:30-12:30:

**Izhar Oppenheim,**Introduction to Random Groups

14:00-16:00:

**Izhar Oppenheim,**Property (T) and hyperbolicity for random groups in the permutation model

2018
Jan
31

2018
Jun
21

# Colloquium: Erdos lecture - Canceled

2:30pm to 3:30pm

## Location:

Manchester Building (Hall 2), Hebrew University Jerusalem

Given a convex polytope P, what is the number of integer points in P? This problem is of great interest in combinatorics and discrete geometry, with many important applications ranging from integer programming to statistics. From a computational point of view it is hopeless in any dimensions, as the knapsack problem is a special case. Perhaps surprisingly, in bounded dimension the problem becomes tractable. How far can one go? Can one count points in projections of P, finite intersections of such projections, etc.?

2016
Jun
09

# Joint Amitsur Algebra&NT Seminar: Shai Haran (Technion), "New foundations for geometry"

12:00pm to 1:15pm

## Location:

Manchester Building (Ross 63), Jerusalem, Israel

*** Please note the LOCATION ***

We shall give a simple generalization of commutative rings. The

category GR of such generalized rings contains ordinary commutative

rings (fully, faithfully), but also the "integers" and the "residue

field" at a real or complex place of a number field ; the "field with

one element" F1 (the initial object of GR) ; the "Arithmetical

Surface" (the categorical sum of the integers Z with them self). We

shall show this geometry sees the real and complex places of a number

We shall give a simple generalization of commutative rings. The

category GR of such generalized rings contains ordinary commutative

rings (fully, faithfully), but also the "integers" and the "residue

field" at a real or complex place of a number field ; the "field with

one element" F1 (the initial object of GR) ; the "Arithmetical

Surface" (the categorical sum of the integers Z with them self). We

shall show this geometry sees the real and complex places of a number

2018
Jan
25

2018
Jun
28

# Colloquium: Barry Simon (Caltech) - "More Tales of our Forefathers"

2:30pm to 3:30pm

## Location:

Manchester Building (Hall 2), Hebrew University Jerusalem

This is not a mathematics talk but it is a talk for mathematicians. Too often, we think of historical mathematicians as only names assigned to theorems. With vignettes and anecdotes, I'll convince you they were also human beings and that, as the Chinese say, "May you live in interesting times" really is a curse. Among the mathematicians with vignettes are Riemann, Newton, Poincare, von Neumann, Kato, Loewner, Krein and Noether. This talk is in two parts. The second part will be given from 4:00 to 5:00 (not 5:30) in the Basic Notions seminar.

2016
Apr
21

# Amitsur Algebra: Konstantin Golubev (HU)

12:00pm to 1:15pm

## Location:

Manchester Building (room 209), Jerusalem, Israel

Title: Spectral approach to the chromatic number of a simplicial complex

Abstract: In this talk, we'll summarize results obtained in recent years in a pursuit for spectral bounds for the chromatic number of a simplicial complex. As the principal application, we'll show that Ramanujan complexes serve as family of explicitly constructed complexes with large girth and large chromatic number. We'll also present other results, such as a bound on the expansion and a bound on the mixing of a complex, and refer to open questions.

Abstract: In this talk, we'll summarize results obtained in recent years in a pursuit for spectral bounds for the chromatic number of a simplicial complex. As the principal application, we'll show that Ramanujan complexes serve as family of explicitly constructed complexes with large girth and large chromatic number. We'll also present other results, such as a bound on the expansion and a bound on the mixing of a complex, and refer to open questions.

2018
Jun
14

# Colloquium - Zuchovitzky lecture: Lior Yanovski (HUJI) "Homotopy cardinality and the l-adic analyticity of Morava-Euler characteristic"

2:30pm to 3:30pm

## Location:

Manchester Building (Hall 2), Hebrew University Jerusalem

A finite set has an interesting numerical invariant - its cardinality. There are two natural generalizations of "cardinality" to a (homotopy) invariant for (suitably finite) spaces. One is the classical Euler characteristic. The other is the Baez-Dolan "homotopy cardinality". These two invariants, both natural from a certain perspective, seem to be very different from each other yet mysteriously connected. The question of the precise relation between them was popularized by John Baez as one of the "mysteries of counting".

2018
Jan
25

# Special seminar: Sylvian Cappell (NYU) "What can be the fixed point sets of a given finite group acting on a non-simply-connected compact space?"

10:30am to 11:30am

## Location:

Ross 70

We will report on a joint work with Shmuel Weinberger of U. of Chicago & Min Yan of Hong Kong U. of Sci. & Tech.

2016
Jan
07

# Amitsur Algebra: Gili Schul (Hebrew U): Rapid expansion in finite simple groups

12:00pm to 1:15pm

## Location:

Manchester Building (room 209), Jerusalem, Israel

Title: Rapid expansion in finite simple groups

Abstract: We show that small normal subsets $A$ of finite simple groups expand

very rapidly -- namely, $|A^2| \ge |A|^{2-\epsilon}$, where $\epsilon >0$ is

arbitrarily small.

Joint work with M. W. Liebeck and A. Shalev

Abstract: We show that small normal subsets $A$ of finite simple groups expand

very rapidly -- namely, $|A^2| \ge |A|^{2-\epsilon}$, where $\epsilon >0$ is

arbitrarily small.

Joint work with M. W. Liebeck and A. Shalev

2018
Apr
22

# Geometric, Topological and Computational Aspects of High-Dimensional Combinatorics

Sun, 22/04/2018 (All day) to Thu, 26/04/2018 (All day)

## Location:

Israel Institute for Advanced Studies, The Hebrew University of Jerusalem

2018
Feb
13

2016
Jun
16

# Amitsur Algebra: Gili Golan, "The generation problem in Thompson group F"

12:00pm to 1:15pm

## Location:

Manchester Building (room 209), Jerusalem, Israel

We show that the generation problem in Thompson group F is decidable, i.e., there is an algorithm which decides if a finite set of elements of F generates the whole F. The algorithm makes use of the Stallings 2-core of subgroups of F, which can be defined in an analogue way to the Stallings core of subgroups of a free group. An application of the algorithm shows that F is a cyclic extension of a group K which has a maximal elementary amenable subgroup B. The group B is a copy of a subgroup of F constructed by Brin.

2016
Apr
07

# Amitsur Algebra: Ayala Byron (HUJI), "Definable fields in the free group"

12:00pm to 1:15pm

## Location:

Manchester Building (room 209), Jerusalem, Israel

Abstract: In the early 2000s Sela proved that all non-abelian free groups share a common first-order theory. Together with R. Sklinos, we use tools developed in his work to show that no infinite field is definable in this theory. In this talk we will survey the line of proof for a formal solution theorem for a simple sort of definable sets, that have a structure of a hyperbolic tower, and use it to characterize definable sets that do not carry a definable structure of an abelian group.