Seminars

2016 Dec 12

Combinatorics: Zur Luria (ETH)

11:00am to 1:00pm

Location: 

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

Combinatorics seminar: Eric Babson

11:00am to 12:30pm

Location: 

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

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.
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 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 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 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.

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