2018
Jan
22

# Combinatorics Seminar: Uli Wagner

11:00am to 12:30pm

## Location:

Room 130, Feldman Building

Title: Shellability is NP-complete

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2018
Jan
22

11:00am to 12:30pm

Room 130, Feldman Building

Title: Shellability is NP-complete

2017
Mar
20

11:00am to 1:00pm

Rothberg B220 (CS bldg)

Speaker: Doron Puder, TAU
Title: Meanders and Non-Crossing Partitions
Abstract: Imagine a long river and a closed (not self-intersecting) racetrack that crosses the river by bridges 2n times. This is called a meander. How many meanders are there with 2n bridges (up to homeomorphisms of the plane that stabilizes the river)? This challenging question, which is open for several decades now, has connections to several fields of mathematics.

2016
Jun
16

11:00am to 12:00pm

Rothberg A410

Speaker: Bram Petri, Max Planck Institute, Bonn
Title: The length spectrum of a random surface

2017
Nov
13

11:00am to 12:30pm

2016
Dec
12

11:00am to 1:00pm

B220 Rothberg (CS)

Speaker: Zur Luria (ETH)
Title: Hamiltonian spheres in random hypergraphs
Abstract:
Hamiltonian cycles are a fundamental object in graph theory, and combinatorics in general. A classical result states that in the random graph model G(n,p), there is a sharp threshold for the appearance of a Hamiltonian cycle. It is natural to wonder what happens in higher dimensions - that is, in random uniform hypergraphs?

2017
Nov
06

11:00am to 12:30pm

130 at the IIAS

Title: Gaussian Random Links
Abstract: A model for random links is obtained by fixing an
initial curve in some n-dimensional Euclidean space and
projecting the curve on to random 3 dimensional subspaces. By
varying the curve we obtain different models of random
knots, and we will study how the second moment of the average crossing
number change as a function of the initial curve.
This is based on work of Christopher Westenberger.

2017
Feb
27

10:30am to 12:30pm

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

11:00am to 1:00pm

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

11:00am to 1:00pm

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

11:00am to 12:30pm

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

11:00am to 1:00pm

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

11:00am to 12:30pm

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

11:00am to 1:00pm

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

11:00am to 1:00pm

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

11:00am to 12:30pm

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.