2017 Dec 04

# Combinatorics Seminar: Anna Gundert

11:00am to 12:30pm

## Location:

Room 130 IIAS
Title:
The Theta Number of Simplicial Complexes
Abstract:
The celebrated Lovász theta number of a graph is an efficiently computable upper bound for the independence number of a graph, given by a semidefinite program. This talk presents a generalization of the theta number to simplicial complexes of arbitrary dimension, based on real simplicial cohomology theory, in particular higher dimensional Laplacians.
2017 Jan 16

# Combinatorics: Yinon Spinka (TAU) "Long-range order in random colorings and random graph homomorphisms in high dimensions"

11:00am to 1:00pm

## Location:

Rothberg B220 (CS bldg)
Speaker: Yinon Spinka (TAU)
Title: Long-range order in random colorings and random graph homomorphisms in high dimensions
Abstract:
2017 Nov 27

# Combinatorics Seminar: Maria Hempel

11:00am to 12:30pm

## Location:

Room 130 at the IIAS
Title: Labeling and Eliminating Geometric Realization Spaces
Abstract: I will introduce a Moduli-space of Shapes of Polyhedra, and show how they may be eliminated by labeling their underlying combinatorial data. I discuss how this relates to geometric realization problems and in particular to flexibility of polyhedra.
2017 Apr 23

# Combinatorics: Amitay Kamber (HU) " Lp Expander Complexes"

11:00am to 1:00pm

## Location:

Rothberg B221 (CS building)
Speaker: Amitay Kamber, HU
Title: Lp Expander Complexes.
Abstract: In recent years, several different notions of high dimensional expanders have been proposed (which in general are not equivalent), each with its own goal and motivation. The goal of this talk is to propose another generalization, based on ideas from the representation theory of p-adic groups.
By comparing a complex to its universal cover, we show how to define Lp-expanders and in particular L2-expanders, which are Ramanujan complexes, generalizing the notions of expander graphs and Ramanujan graphs.
2016 Nov 14

# Combinatorics: Andrew Thomason (Cambridge) "Hypergraph containers"

11:00am to 1:00pm

## Location:

Rothberg (CS) B220
Speaker: Andrew Thomason, Cambridge
Title: Hypergraph containers
Abstract:
A collection of containers for a (uniform) hypergraph is a collection of
subsets of the vertex set such that every independent set lies inside a
container. It has been discovered (Balogh, Morris, Samotij, and Saxton)
that it is always possible to find such a collection for which the
containers are not big (close to independent) and the size of the
collection itself is quite small. This basic, if surprising, fact has many
2017 Oct 23

# Combinatorics seminar: Quang Nhat Le

11:00am to 12:30pm

## Location:

Eilat Hall at the IIAS
Title: Counting lattice points inside a d-dimensional polytope via Fourier analysis
Abstract: Given a convex body $B$ which is embedded in a Euclidean space $R^d$, we can ask how many lattice points are contained inside $B$, i.e. the number of points in the intersection of $B$ and the integer lattice $Z^d$. Alternatively, we can count the lattice points inside B with weights, which sometimes creates more nicely behaved lattice-point enumerating functions.
2016 Dec 26

# Combinatorics: Adam Sheffer (CalTech) "Geometric Incidences and the Polynomial Method"

10:00am to 11:45am

## Location:

B220 Rothberg (CS)
Title: Geometric Incidences and the Polynomial Method
Abstract: While the topic of geometric incidences has existed for several decades, in recent years it has been experiencing a renaissance due to the introduction of new polynomial methods. This progress involves a variety of new results and techniques, and also interactions with fields such as algebraic geometry and harmonic analysis.
2018 Jan 15

# Combinatorics Seminar: Leonardo Martinez

11:00am to 12:30pm

2017 Mar 27

# Combinatorics: Micha Sharir (TAU) "Eliminating depth cycles for lines and triangles, with applications to bounding incidences"

11:00am to 1:00pm

## Location:

Rothberg B220 (CS bldg)
Speaker: Micha Sharir (Tel Aviv University)
Title: Eliminating depth cycles for lines and triangles, with applications to bounding incidences
Abstract:
---------
The talk presents three related results.
We first consider the problem of eliminating all depth cycles in a set of n lines in 3-space.
For two lines l_1, l_2 in 3-space (in general position), we say that l_1 lies below l_2 if the
unique vertical line that meets both lines meets l_1 at a point below the point where it meets l_2.
2016 Oct 31

# Combinatorics: Gil kalai (HU) "Algebraic-topological invariants of hypergraphs and extremal combinatorics"

11:00am to 1:00pm

## Location:

Rothberg (CS) B220
Speaker: Gil Kalai, HU
Title: Algebraic-topological invariants of hypergraphs and extremal combinatorics
Abstract:
We will discuss some algebraic invariants of hypergraphs and some connection to algebraic topology. We will present some conjectural (rather speculative) relations with two central problems in extremal combinatorics: The Turan (4,3) conjecture and the Erdos-Rado sunflower conjecture.
2016 Dec 19

# Combinatorics: Lukas Kühne (U. Bonn) "Heavy hyperplanes in multiarrangements and their freeness"

11:00am to 1:00pm

## Location:

Rothberg B220 (CS)
Speaker: Lukas Kühne (University of Bonn)
Title: Heavy hyperplanes in multiarrangements and their freeness
Abstract:
One of the central topics among the theory of hyperplane arrangements is their freeness. Terao's conjecture tries to link the freeness with the combinatorics of an arrangement. One of the few categories of arrangements which satisfy this conjecture consists of 3-dimensional arrangements with an unbalanced Ziegler restriction. This means that the arrangement contains a lot of hyperplanes intersecting in one single line
2017 Nov 20

# Combinatorics seminar:Sria Louis

11:00am to 12:30pm

## Location:

IIAS Room 130
Speaker 1: Sria Louis
Title: Asymptotically Almost Every 2r-regular Graph has an Internal Partition
Abstract: An internal partition of a graph is a partitioning of the vertex set into two parts such that for every vertex, at least half of its neighbors are on its side. It is easy to notice that such a partition doesn't always exist (e.g. - cliques), though, both the existence and finding of such a partition - are open problems.
2017 Mar 06

# Combinatorics: Zilin Jiang (Technion) "Relations between Tverberg points and central points"

11:00am to 1:00pm

## Location:

Rothberg B220 (CS)
Speaker: Zilin Jiang (Technion)
Title: Relations between Tverberg points and central points
Abstract:
Given 3n lines in general position in the plane, it is always possible to partition them into n triples of lines so that all the triangles, formed by the triples, share a common point. This result is known back in 1988 by J.P. Roudneff. Strangely, in higher dimensions, it is only proved by Roman Karasev for n that is a prime power.
2016 Feb 29

# Combinatorics

Repeats every week every Monday until Mon Jun 13 2016 .
10:30am to 12:30pm

10:30am to 12:30pm
10:30am to 12:30pm
10:30am to 12:30pm
10:30am to 12:30pm
10:30am to 12:30pm
10:30am to 12:30pm
10:30am to 12:30pm
10:30am to 12:30pm
10:30am to 12:30pm
10:30am to 12:30pm
10:30am to 12:30pm
10:30am to 12:30pm
10:30am to 12:30pm
10:30am to 12:30pm
10:30am to 12:30pm

## Location:

B220 Rothberg (CS and Engineering building)
NOTE THE SPECIAL TIME: 11:00--12:30
Speaker: Eyal Ackerman, University of Haifa at Oranim
Title: Coloring points with respect to squares
Abstract:
Is there an absolute constant $m$ such that any finite planar point set can be 2-colored such that every axis-parallel square that contains at least $m$ points contains points of both colors? I will discuss this problem and related ones.