2017 Feb 27

# Combinatorics: Thilo Weinert (BGU) "The Ramsey Theory of ordinals and its relation to finitary combinatorics"

10:30am to 12:30pm

## Location:

Rothberg B220 (CS bldg)
Speaker: Thilo Weinert, BGU Title: The Ramsey Theory of ordinals and its relation to finitary combinatorics Abstract: Ramsey Theory is a branch of mathematics which is often times summed up by the slogan “complete disorder is impossible”. An important branch of finitary Ramsey Theory lies in the determination of Ramsey Numbers. Infinitary Ramsey Theory on the other hand is an important branch of set theory. Whereas the Ramsey Theory of the Uncountable often times features independence phenomena, the Ramsey Theory of the Countably Infinite provides many interesting combinatorial challenges.
2017 Jun 18

# Combinatorics: Ehud Fridgut (Weizmann Institute) "Almost-intersecting families are almost intersecting-families."

11:00am to 1:00pm

## Location:

Rothberg B221 (CS building)
Speaker: Ehud Fridgut (Weizmann Institute) Title: Almost-intersecting families are almost intersecting-families. Abstract: Consider a family of subsets of size k from a ground set of size n (with k < n/2). Assume most (in some well defined sense) pairs of sets in the family intersect. Is it then possible to remove few (in some well defined sense) sets, and remain with a family where every two sets intersect? We will answer this affirmatively, and the route to the answer will pass through a removal lemma in product graphs.
2016 Nov 28

# Combinatorics: Elon Lindenstrauss (HU) "On the spectral gap of Cayley graphs for the group of affine transformations"

11:00am to 1:00pm

## Location:

B220 Rothberg (CS building)
Speaker: Elon Lindenstrauss (HU) Title: On the spectral gap of Cayley graphs for the group of affine transformations Abstract: Joint work with P. Varju. One of the most important open questions regarding spectral gap of Cayley graphs is the dependence of the gap on the choice of generators.
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 applications, and allows easy proofs of some theorems that were previously difficult or unknown.
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)
Speaker: Adam Sheffer, CalTech 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. This depth relationship typically has cycles, which can be eliminated if we cut the lines into
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