## Seminars and Colloquia by Series

Friday, April 14, 2017 - 15:05 , Location: Skiles 005 , Glenn Hurlbert , Virginia Commonwealth University , , Organizer: Heather Smith
The fundamental EKR theorem states that, when n≥2r, no pairwise intersecting family of r-subsets of {1,2,...,n} is larger than the family of all r-subsets that each contain some fixed x (star at x), and that a star is strictly largest when n>2r. We will discuss conjectures and theorems relating to a generalization to graphs, in which only independent sets of a graph are allowed. In joint work with Kamat, we give a new proof of EKR that is injective, and also provide results on a special class of trees called spiders.
Friday, April 7, 2017 - 15:55 , Location: Skiles 005 , , Cornell University , , Organizer: Megan Bernstein
The flow polytope associated to an acyclic graph is the set of all nonnegative flows on the edges of the graph with a fixed netflow at each vertex. We will examine flow polytopes arising from permutation matrices, alternating sign matrices and Tesler matrices. Our inspiration is the Chan-Robins-Yuen polytope (a face of the polytope of doubly-stochastic matrices), whose volume is equal to the product of the first n Catalan numbers (although there is no known combinatorial proof of this fact!). The volumes of the polytopes we study all have nice product formulas.
Friday, April 7, 2017 - 15:05 , Location: Skiles 005 , Lionel Levine , Cornell University , Organizer: Megan Bernstein
The theme of this talk is walks in a random environment of "signposts" altered by the walker. I'll focus on three related examples: 1. Rotor walk on Z^2. Your initial signposts are independent with the uniform distribution on {North,East,South,West}. At each step you rotate the signpost at your current location clockwise 90 degrees and then follow it to a nearest neighbor. Priezzhev et al. conjectured that in n such steps you will visit order n^{2/3} distinct sites. I'll outline an elementary proof of a lower bound of this order. The upper bound, which is still open, is related to a famous question about the path of a light ray in a grid of randomly oriented mirrors. This part is joint work with Laura Florescu and Yuval Peres. 2. p-rotor walk on Z. In this walk you flip the signpost at your current location with probability 1-p and then follow it. I'll explain why your scaling limit will be a Brownian motion perturbed at its extrema. This part is joint work with Wilfried Huss and Ecaterina Sava-Huss. 3. p-rotor walk on Z^2. Rotate the signpost at your current location clockwise with probability p and counterclockwise with probability 1-p, and then follow it. This walk “organizes” its environment of signposts. The stationary environment is an orientation of the uniform spanning forest, plus one additional edge. This part is joint work with Swee Hong Chan, Lila Greco and Boyao Li.
Monday, March 27, 2017 - 15:05 , Location: Skiles 005 , Damir Yeliussizov , UCLA , , Organizer: Prasad Tetali
I will talk about the problem of computing the number of integer partitions into parts lying in some integer sequence. We prove that for certain classes of infinite sequences the number of associated partitions of an input N can be computed in time polynomial in its bit size, log N. Special cases include binary partitions (i.e. partitions into powers of two) that have a key connection with Cayley compositions and polytopes. Some questions related to algebraic differential equations for partition sequences will also be discussed. (This is joint work with Igor Pak.)
Friday, March 10, 2017 - 15:05 , Location: Skiles 005 , , Georgia Tech , Organizer: Fidel Barrera-Cruz
Researchers here at Georgia Tech initiated a "Ramsey Theory" on binary trees and used the resulting tools to show that the local dimension of a poset is not bounded in terms of the tree-width of its cover graph. Subsequently, in collaboration with colleagues in Germany and Poland, we extended these Ramsey theoretic tools to solve a problem posed by Seymour. In particular, we showed that there is an infinite sequence of graphs with bounded tree-chromatic number and unbounded path-chromatic number. An interesting detail is that our research showed that a family conjectured by Seymour to have this property did not. However, the insights gained in this work pointed out how an appropriate modification worked as intended. The Atlanta team consists of Fidel Barrera-Cruz, Heather Smith, Libby Taylor and Tom Trotter The European colleagues are Stefan Felsner, Tamas Meszaros, and Piotr Micek.
Friday, March 3, 2017 - 15:05 , Location: Skiles 005 , , Adam Mickiewicz University , , Organizer: Lutz Warnke
In the talk we state, explain, comment, and finally prove a theorem (proved jointly with Yuval Peled) on the size and the structure of certain homology groups of random simplicial complexes. The main purpose of this presentation is to demonstrate that, despite topological setting, the result can be viewed as a statement on Z-flows in certain model of random hypergraphs, which can be shown using elementary algebraic and combinatorial tools.
Friday, February 24, 2017 - 15:05 , Location: Skiles 005 , , Dartmouth College , , Organizer: Torin Greenwood
In enumerative combinatorics, it is quite common to have in hand a number of known initial terms of a combinatorial sequence whose behavior you'd like to study. In this talk we'll describe two techniques that can be used to shed some light on the nature of a sequence using only some known initial terms. While these methods are, on the face of it, experimental, they often lead to rigorous proofs. As we talk about these two techniques -- automated conjecturing of generating functions, and the method of differential approximation -- we'll exhibit their usefulness through a variety of combinatorial topics, including matchings, permutation classes, and inversion sequences.
Friday, February 17, 2017 - 15:05 , Location: Skiles 005 , , University of Louisville , Organizer: Fidel Barrera-Cruz
Many classical hard algorithmic problems on graphs, like coloring, clique number, or the Hamiltonian cycle problem can be sped up for planar graphs resulting in algorithms of time complexity $2^{O(\sqrt{n})}$. We study the coloring problem of unit disk intersection graphs, where the number of colors is part of the input. We conclude that, assuming the Exponential Time Hypothesis, no such speedup is possible. In fact we prove a series of lower bounds depending on further restrictions on the number of colors. Generalizations for other shapes and higher dimensions were also achieved. Joint work with E. Bonnet, D. Marx, T. Miltzow, and P Rzazewski.
Friday, February 10, 2017 - 15:05 , Location: Skiles 005 , Bartosz Walczak , Jagiellonian University in Kraków , , Organizer: Heather Smith
A class of graphs is *χ-bounded* if the chromatic number of all graphs in the class is bounded by some function of their clique number. *String graphs* are intersection graphs of curves in the plane. Significant research in combinatorial geometry has been devoted to understanding the classes of string graphs that are *χ*-bounded. In particular, it is known since 2012 that the class of all string graphs is not *χ*-bounded. We prove that for every integer *t*≥1, the class of intersection graphs of curves in the plane each of which crosses a fixed curve *c* in at least one and at most *t* points is *χ*-bounded. This result is best possible in several aspects; for example, the upper bound *t* on the number of crossings of each curve with *c* cannot be dropped. As a corollary, we obtain new upper bounds on the number of edges in so-called *k*-quasi-planar topological graphs. This is joint work with Alexandre Rok.
Friday, February 3, 2017 - 15:05 , Location: Skiles 005 , , Georgia Tech , , Organizer: Torin Greenwood
A graphical model encodes conditional independence relations via the Markov properties. For an undirected graph these conditional independence relations are represented by a simple polytope known as the graph associahedron, which can be constructed as a Minkowski sum of standard simplices. There is an analogous polytope for conditional independence relations coming from any regular Gaussian model, and it can be defined using relative entropy.  For directed acyclic graphical models we give a construction of this polytope as a Minkowski sum of matroid polytopes.  The motivation came from the problem of learning Bayesian networks from observational data.  This is a joint work with Fatemeh Mohammadi, Caroline Uhler, and Charles Wang.