2019 Jun 03

# Combinatorics - back to back:

11:00am to 1:00pm

## Location:

CS Rothberg bldg, room B-500, Safra campus
First talk: Speaker: Madeleine Weinstein (Berkeley) Title: Voronoi Cells of Varieties Abstract:
2019 May 06

# Combinatorics: Omri Ben Eliezer (TAU) "Finding patterns in permutations"

11:00am to 1:00pm

## Location:

CS building, room B-500, Safra campus
Speaker: Omri Ben Eliezer, TAU Title: Finding patterns in permutations Abstract: For two permutations sigma and pi, we say that sigma contains a copy of pi, if there is a subset (not necessarily consecutive) of elements in sigma, whose relative order is the same as in pi. For example, if pi = (1,2,3), then a copy of pi in sigma amounts to an increasing subsequence in sigma of length 3. As shown by Guillemot and Marx, a copy of a constant length pi can be found in sigma in linear time. However, how quickly can one find such a
2019 Jun 10

# Combinatorics: Eyal Karni (BIU) "Combinatorial high dimensional expanders"

11:00am to 1:00pm

## Location:

CS bldg, room B-500, Safra campus
Speaker: Eyal Karni (BIU) Title: Combinatorial high dimensional expanders Abstract: An eps-expander is a graph G=(V,E) in which every set of vertices X where |X|<=|V|/2 satisfies |E(X,X^c)|>=eps*|X| . There are many edges that "go out" from any relevant set.
2019 May 27

# Combinatorics: Uri Rabinovich (U. Haifa) "SOME EXTREMAL PROBLEMS ABOUT SIMPLICIAL COMPLEXES"

11:00am to 1:00pm

## Location:

CS Rothberg bldg, room B-500, Safra campus
Speaker: Uri Rabinovich (U. Haifa) Title: SOME EXTREMAL PROBLEMS ABOUT SIMPLICIAL COMPLEXES Abstract: We shall discuss the following three issues: * The existence of Hamiltonian d-cycles, i.e., simple d-cycles containing a spanning d-hypertree of a complete d-complex K_n^d; * The existence of a distribution D over spanning d-hypertrees T of K_n^d, so that for ANY (d-1)-cycle C there, the expected size of the d-filling of C with respect to a random T from D is Omega(n^d); * The existence of f(k,d) such that any d-simplex of rank r with > f(k,d)*r d-faces contains
2019 Jun 24

# Combinatorics: Doron Puder (TAU) "Aldous' spectral gap conjecture for normal sets"

11:00am to 1:00pm

## Location:

CS bldg, room B-500, Safra campus
Speaker: Doron Puder, TAU Title: Aldous' spectral gap conjecture for normal sets Abstract: Aldous' spectral gap conjecture, proved in 2009 by Caputo, Liggett and Richthammer, states the following a priori very surprising fact: the spectral gap of a random walk on a finite graph is equal to the spectral gap of the interchange process on the same graph.
2019 May 13

# Combinatorics: Shira Zerbib (U. Michigan, Iowa State University) "Envy-free division of a cake without the “hungry players" assumption"

11:00am to 1:00pm

## Location:

CS bldg, room B-500, Safra campus
Speaker: Shira Zerbib (U. Michigan, Iowa State University) Title: Envy-free division of a cake without the “hungry players" assumption Abstract: The fair division theorem due to Stromquist (1980) ensures that under some conditions it is possible to divide a rectangular cake into n pieces and assign one piece to each of n players such that no player strictly prefers a piece that has not been assigned to him.
2019 Apr 01

# Combinatorics: Raphy Yuster (U. Haifa) "On some Ramsey type problems in tournaments"

11:00am to 1:00pm

## Location:

CS B-500, Safra campus
Speaker: Raphy Yuster, U. Haifa Title: On some Ramsey type problems in tournaments Abstract: I will talk about several Ramsey type problems in tournaments guaranteeing the existence of subgraphs with certain chromatic properties. Here are two such problems which attracted some attention recently: 1. Let g(n) be the smallest integer such that every tournament with more than g(n) vertices has an *acyclic subgraph* with chromatic number larger than n. It is straightforward that Omega(n) <= g(n) <= n^2. An improvement of both bounds is presented.
2019 Mar 18

# Combinatorics: Arindam Banerjee (RMVERI), TBA

11:00am to 1:00pm

## Location:

CS B-500, Safra campus
Speaker: Arindam Banerjee, RMVERI Title: Castelnuovo-Mumford Regularity, Combinatorics and Edge Ideals. Abstract:
2019 Jun 17

# NO seminar: mini conference in memory of Prof. Yossi Zaks at U. Haifa

11:00am to 1:00pm

## Location:

U. Haifa
From Raphy Yuster: On Monday 17 June, 2019 we will hold a one day mini conference in memory of Professor Yossi Zaks (see attached poster or updated information in http://sciences.haifa.ac.il/math/wp/?page_id=1382 ) Mini conference: Yossi Zaks Memorial Meeting – Monday, June 17, 2019 list of speakers Noga Alon, Princeton University and Tel Aviv University Gil Kalai, Hebrew University Nati Linial, Hebrew University Rom Pinchasi, The Technion Organizers Raphael Yuster, University of Haifa
2019 Apr 29

# Combinatoric: Karthik C. Srikanta (Weizmann Institute) "On Closest Pair Problem and Contact Dimension of a Graph"

11:00am to 1:00pm

## Location:

CS B-500, Safra campus
Speaker: Karthik C. Srikanta (Weizmann Institute) Title: On Closest Pair Problem and Contact Dimension of a Graph Abstract: Given a set of points in a metric space, the Closest Pair problem asks to find a pair of distinct points in the set with the smallest distance. In this talk, we address the fine-grained complexity of this problem which has been of recent interest. At the heart of all our proofs is the construction of a family of dense bipartite graphs with special embedding properties and are inspired by the construction of locally dense codes.
2019 Apr 08

# Combinatorics: Kim Minki (Technion) "The fractional Helly properties for families of non-empty sets"

11:00am to 1:00pm

## Location:

CS B-500, Safra campus
Speaker: Kim Minki, Technion Title: The fractional Helly properties for families of non-empty sets Abstract: Let $F$ be a (possibly infinite) family of non-empty sets. The Helly number of $F$ is defined as the greatest integer $m = h(F)$ for which there exists a finite subfamily $F'$ of cardinality $m$ such that every proper subfamily of $F'$ is intersecing and $F'$ itself is not intersecting. For example, Helly's theorem asserts that the family of all convex sets in $d$-dimensional Euclidean space has Helly number $d+1$.
2019 Mar 25

# Combinatorics: Roy Meshulam (Technion) "Topology and combinatorics of the complex of flags"

11:00am to 1:00pm

## Location:

CS B-500, Safra campus
Speaker: Roy Meshulam (Technion) Title: Topology and combinatorics of the complex of flags Abstract:
2019 May 20

# Combinatorics: Rom Pinchasi (Technion) TBA

11:00am to 1:00pm

## Location:

CS B-500, Safra campus
2019 Apr 15

# Combinatorics: problem session

11:00am to 1:00pm

## Location:

CS B-500, Safra campus
Speaker: Eyal Karni, BIU Title: Combinatorial high dimensional expanders Abstract: An eps-expander is a graph G=(V,E) in which every set of vertices X where |X|<=|V|/2 satisfies |E(X,X^c)|>=eps*|X| . There are many edges that "go out" from any relevant set.
2018 Dec 24

# Combinatorics: Benny Sudakov, ETH, TBA

11:00am to 1:00pm

## Location:

Rothberg CS room B500, Safra campus, Givat Ram
Speaker: Benny Sudakov, ETH, Zurich Title: Subgraph statistics Abstract: Consider integers $k,\ell$ such that $0\le \ell \le \binom{k}2$. Given a large graph $G$, what is the fraction of $k$-vertex subsets of $G$ which span exactly $\ell$ edges? When $G$ is empty or complete, and $\ell$ is zero or $\binom k 2$, this fraction can be exactly 1. On the other hand if $\ell$ is not one these extreme values, then by Ramsey's theorem, this fraction is strictly smaller than 1. The systematic study of the above question was recently initiated by